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Billy Katagiri (ビリー・カタギリ Birī Katagiri?) is a fictional character in Mobile Suit Gundam 00. Billy is an MS engineer for the Union's MSWΛD in Season 1. In season 2, he becomes A-Laws' chief technical engineer to develop new generation mobile weapons for the ESF and the Innovators.

Personality & Character

Billy is overall a sentimental, passionate, romantic, and caring person for those important in his life. After learning Leesa (Sumeragi Lee Noriega) is part of Celestial Being, his emotions had taken a negative change. He became spiteful, vengeful, and discontent how things turned out between him and Sumeragi. He has since poured all his feelings into MS development to help cope with his anger/hatred against Celestial Being. By the end of AD 2312 (season 2), after reconciling with Leesa, he reverted to his original personality and continues to devote his life into MS advancement.

Skills & Abilities

Billy is a highly skilled scientist of multiple disciplines for mobile suit development. He's an advanced physicist, mechanical/robotic engineer, intelligence investigator, and a leading authority in GN-tech. Before his MSWAD days, he used to gather intelligence to help the Union maintain an edge in their technological developments. After the appearance of Celestial Being, he devoted the latter of his life into R&D for advanced MS development. After understanding the nature of the GN Drives, he became one of the leading authorities in MS development with GN-Tech.


Early Days

Graduate School

Before Billy's days as a Union Engineer, he was a college student studying at the Union International University under Ralph Eifman. While studying, he encountered the beautiful prodigy, Leesa Kujo, and fell in love with her. While they remained as friends, he always desired more than friendship. Due to Leesa's quick advancement in military studies, he was saddened to know that she left for the AEU military; they maintained communications throughout all this time.

AEU Incident & Joining Union

Sometime before AD 2306 (when Sumeragi joined Celestial Being), Billy and Eifman learned of her unfortunate incident involving AEU military forces suffering casualties in a friendly fire incident. After graduation, Billy continued to work with Prof Eifman and kept in touch with Leesa. He later joined the Union military as an intelligence officer as well as a MS technical engineer. In between those years, he befriended Union ace pilot, Graham Aker.

AEU Enact Debut & Celestial Being

The AEU recently completed a brand new MS series, the AEU-09 AEU Enact. Various politicians and military dignitaries came to see the new high performance Enact, including Union's Billy Katigiri. Billy was there to evaluate the capabilities of the Enact against Union's new Flag series. As Billy watched, he commented out loud, "So this is the Enact. The first AEU mobile suit that runs on solar energy?" Graham overheard Billy and commented, "The development of the AEU's orbital elevator is definitely lagging. They probably want to make up for that by making their mobile suit state of the art." Billy asked, "Well look at this, should the ace of MSWAD be showing his face around here?" to which Graham answered, " Of course I shouldn't be here." Billy continued, "You know the AEU's got some nerve alright, announcing their new model at the same time as the Human Reform League's 10th anniversary ceremony. " Graham retorted, "So what do you think of this new model anyway?" Billy commented on the Enact "To be perfectly honest, it's just a knock off of our Flag model. Only the exterior design is original." Just then, AEU ace pilot Patrick Colasour overheard and shouted, "You there, I can hear you. What did you just say? Well come on!" Graham comments, "Well at least it's got good sound pick up." Billy laughed and said, "I guess so." Moments later, an unknown white mobile suit arrives to the demonstration. As the suit descends to the demonstration area, Graham and the rest of the demonstration gradually begin noticing it coming.

Billy looks up to the descending MS and says, "A mobile suit? Incredible, I didn't know they had another new model?" Graham says, "That's not theirs...What is with that light..." Spectators attempted to reach contact with AEU command and Patrick/Enact, but all communications were jammed. Billy wondered, "It's not an AEU machine, then whose is it?" Billy continued to watch as Patrick/Enact challenged the unknown MS to melee combat. To everyone's surprise, the unknown-MS quickly cut off the Enact's left arm and quickly sliced the unit to the ground. Graham wanted a closer look and took an observer's binoculars to lookout for any markings. He then saw the word, GUNDAM, etched to the forehead of the MS. Graham comments, " that the name of the mobile suit or something else?" As the Gundam left, it's unusual propulsion system emitted green glittery light. Graham comments, "Again with that light..." Billy remarks, "How can it fly without a propulsion system?" As Patrick got out of his cockpit, he complaining about the situation. Upon closer observation, Graham remarked, "Wow, that's pretty impressive. I guess the brand new Enact deserves high marks for pilot safety. But that mobile suit is a whole other thing. Was this meant to hamper the AEU's military build up or was this meant to just be a warning. Whatever the case, there's no way the AEU's going to take something like this just lying down." After the end of the assault, Graham and Billy went to their SUV for detailed analysis.

While inside the SUV, Graham and Billy continued to talk about the Gundam. Graham mentioned that the attack was a warning. Billy asked, "So that mobile suit is exposing the AEU's undeclared fighting strength?" Graham replied, "Yeah, it looks to me like it is showing the world the AEU has more military power than is allowed by the treaty limitations. This is a check and a warning." Billy asks, "And why would it want to do that?" Graham answers, "That's something you'll have to ask the pilot of that Gundam thing. But you know, I can't see the AEU keeping quiet about this." As they rode the SUV, news came over about the mysterious attack. As twilight settled to darkness, both Graham and Billy monitored the news as Aeolia Schenberg sent a message to the world:

" I would like to address this statement to every single human being born and raised on earth. We call ourselves simply Celestial Being. We are a private armed organization in possession of the mobile weapon Gundam. The main objective of Celestial Being's activities is to completely eliminate acts of war from this world. We do not act for our own benefit or for personal gain. We have chosen to intervene for the greatest goal of all to rid ourselves of the scourge of war. As of this moment I make this declaration to all humanity. Territory, religion, energy no matter what the reason or excuse if there is an evident act of war being carried out, we will commence intervention with our force. Any country, organization or corporation that promotes war will also be a legitimate target for our intervention. We simply call ourselves Celestial Being. We are an armed organization that was established to eliminate all acts of war from this world. I repeat..."

As Aeolia reinforced the goals of CB, Graham was laughing at the announcement. Graham chuckles, saying, "Now that's rich! They seriously believe that they going to eliminate war through force. Celestial Being, your whole existence is a contraction!" After the announcement, Graham drove near AEU's incomplete orbital elevator in Africa. Billy was working on some computer data when he asked Graham, "Don't you have to get back to the military? They're probably panicking right about now." Graham replies, "I want to find out what that Gundam can do. That machine is something else. It's combat performance is one thing, but when it showed up, radar, communications and electronic devices were all disrupted and it was caused by that light'. Katagiri, what's the story there?" Billy replies, "So far all I know is that it's some sort of special particle. that light is probably caused by some kind of photonic decay phenomenon." Graham comments, "A special particle..." Just then, an unknown car drives up to them and they step out to continue their conversation. Billy continues, "It's not just the particles, I think that machine has some other secrets as well." Graham smirks and says, "I guess I like them. I mean I have more than just a casual interest." A Union officer greets them both to notify them that a carrier, loaded with his SVMS-01 Union Flag, is ready for immediate transport to meet a high ranking Union officer.

Detour to Ceylon

As Billy and Graham flew back to the Union base, military reports were coming in about the CB's impending attack on Ceylon. Billy comments, "Former Sri Lanka, eh? It's certainly a mess, even after the Reform League annexed it ten years ago, even backing the Tamil people." Graham retorts, "Yeah, with the goal of ending the conflict, but the truth is different. The Reform League's goal was to add Ceylon as a stepping stone to the east, assuring their supply of solar energy. The Tamil people just happened to be the strongest in the region at the time. But why did the Reform League get involved in the conflict? They were afraid of the region falling out of anarchy." With a momentary thought, Graham then had the pilot alter course for Ceylon and had his Flag prepped for launch. Billy asked, "'Don't tell're being stupid! Graham replied, "Think what you will." As soon as the carrier located Exia, Graham/Flag launched to duel Setsuna F. Seiei/Exia. Graham didn't last long as Exia disarmed his Flag and forced him to retreat.

Back on the carrier, heading back to their original course, Billy and Graham talked about what happened, Billy comments, "Goodness gracious, you're such a troublesome fellow, aren't you?" Graham replies, "I wrecked my rifle, I'll have to write an apology..." Billy retorts, "Don't worry about it, we got some battle data on the Gundam and your Flag was unharmed. Though you didn't obtain the Gundam, we might learn something from the battle worn parts." Graham says, "At least he is young and inexperienced...that Celestial Being pilot." "You saw him?", asked Billy. "No, but you could see it in how he moved his mobile suit.", answered Graham. Just then, an officer announced, "We've lost the Gundam's signal!" Graham comments, "Not like it matters now." Graham and Billy heads to meet the commander of the Union Mobile Suit Warehouse & Development Headquarters (MSWAD).

Anti Celestial Being Campaign

Anti-Gundam Investigative Squad

Billy and Graham had a meeting with the commander of MSWAD. The commander says, "The AEU's new machine being beaten comes as no real surprise, but things have gotten out of hand." Graham comments, "I later encountered that suit, but it was beyond my expectations." Billy says, "With further research, I believe it's possible to defeat it." The commander retorts,"The higher-ups think so too. Since the two of you had contact with the Gundam, you're now receiving a change of orders." Graham reads out-loud, "The Anti-Gundam Investigative Squad', is it?" The commander continues, "It's a new troop division, it was formally created when I handed you those papers. We'll come up with an official name for it later" Billy reads and asks, "Professor Ralph Eifman...the chief of engineering is in charge?" The commander answers, "It looks like that's how important the higher-ups believe this is. This is an urgent situation." Graham salutes and says, "Sir! Lieutenant Graham Aker and Intelligence Officer Billy Katagiri, Anti-Gundam Defense Force positions accepted!" As the two walk through the hallways, Billy says, "This is a surprise. Well, you couldn't have predicted this, could you?" Graham answers, "I'm no god after all. Though I feel I've become a witness to destiny."

Graham's Custom Flag & Taribia

As Billy works on Graham's Flag in the MS hanger, he says, "It's still amazing you were able to return in this suit. The Gundam's output is at least six times that of the Flag's. I wonder what sort of engine it uses?" Graham comments, "The power is fearsome by itself, but with that mobility..." Billy says, "I've checked the battle data. Like I thought, the source of this sort of mobility is photon particles." Graham comments, "Those particles, in addition to the scrambling effects, also play a primary role in suit operation." Just then, Ralph Eifman intervenes, "Not to mention their application to their weapon systems. He's a wicked man. Hording technology decades ahead of our own. If possible, we should capture one. One of those Gundam units" Graham says, "I agree. For that purpose, we need to tune my Flag for the task." Eifman asks, "What about pilot strain?" Graham replies, "It doesn't matter, however, I'd like the job done in one week's time." Eifman smirks and says, "Oh!? You're quite an unreasonable man." Graham says, "If I weren't forceful, I couldn't steal a Gundam." Billy added, "He thinks he's fallen in love with it." Just then, Graham received a call about the Gundams were on the move. Reports came in about an attack at South Africa and Taribia. Graham was eager to intercept the Gundam (Allelujah Haptism/GN-003 Gundam Kyrios); Eifman stopped him. Graham asks, "Why not? One of them is in Taribia. I can get there in time." Eifman replies, "If there's one thing I hate, it's narcotics. If their goal is burning down the drug fields, I'll gladly side with the Gundams. They're severing the sources of wars after all."

A week later, Graham receives his upgraded Flag unit, the SVMS-01E Graham's Union Flag Custom. Eifman tells him the specifications, "I've reinforced the backpack and joints, and I have applied an anti-beam coating to the unit's surface. For weaponry, I procured a new prototype rifle the Iris Corporation." Graham happily replied, "It's magnificent, professor!" Billy continues with the specs, "But, even with the anti-G system activated, you will be feeling about 12Gs when turning at full speed." Graham tells Billy, "That's exactly what I was hoping for." Billy and the remaining group would be introduced to Howard Mason and Daryl Dodge to complete their preliminary group.

Capture Attempt in Republic of Taribia

News sources around the world talked about Taribia succeeding from the Union charter. The Union couldn't let Taribia leave the Union charter as their orbital elevator is stationed at Taribia. Union forces were dispatched to retake Taribia by force (if necessary) to reclaim their elevator. The world's government and forces all presume Celestial Being will appear. Billy learned of CB's attack on Taribia and Taribia's return to the Union's charter through the news.

PMC/AEU Counteroffensive

PMC Trust and the AEU later allied themselves together in attempt to combine their MS forces to defeat and capture the Gundams. As Graham gets his coffee, he says to Billy, "It looks like Moralia is deliberately picking a fight with Celestial Being." Billy retorts, "They've got the AEU backing them now. To finish their Orbital Elevator and begin development of colonies, the AEU has to rely on the assistance of such contractors as well. I'm sure Moralia is also thinking restoring their crumbling economy with this. If their country becomes a battlefield, they'll need the AEU in that situation as well. Also, I'm sure they've thought of getting their hands on one, a Gundam." Graham comments, "Anyway, it looks like we have to sit out this time. Let the AEU's 'ace' handle it."

Reunion with Kujo

In between the impending battle, Billy arranged a meeting with Leesa Kujo at a restaurant bar. As he approached Leesa at the bar, he noticed her watching the news about the PMC/AEU counteroffensive. Billy says to Kujo (Sumeragi), "So you're bothered by this too? Hey..." Bill orders an Evening Piero and continues to talk, "So what do you think about Moralia's actions?" Kujo answers, "Let's not talk about that sort of thing, we haven't seen each other in so long." Billy remarks, "Not since graduate school, eh? I wonder how many years it's been?" Kujo smirks and says, "Don't talk about that, you'll figure out how old I am." Billy laughs and said, "But I already know." Kujo says, "You know, women get older each time they're reminded for their age." Billy said, "'I take it you've collected that data from experience?" Kujo laughs and said, "You haven't changed at all, Billy." Billy retorts, "I'm happy to hear you say so." Billy then told Kujo/Sumeragi about the Union's Anti-Gundam Investigation Force. Kujo remarked, "Anti-Gundam Investigation Force? What's with that weird naming?" Billy says, "It's a new unit, it doesn't have an official name yet." Kujo asked, "And you've been reassigned there?" Billy answers, "Not just me. Well, there are a lot of people...including Professor Eifman." Kujo was surprised to hear about the professor. Billy continues, "The professor's already working on a theory behind the particles emitted by the Gundams." Kujo remarked, "That's something, isn't it? So then, what kind of particles are they?" Billy answers, "That's the question we can never find the answer to." Kujo says, "Really, that's unfortunate." Billy asked, "By the way, what are you up to these days?" Kujo vaguely answered, " lot of stuff" Billy then asked about the infamous AEU incident, "So that thing...?" Kujo answered, "I've already forgotten about it". Billy replies and held her hand, "I see...that's good. I'm glad I was able to meet you." Both later departed after catching up and Billy headed back to MSWAD to observe the impending battle.

Observing the Battle

As the battle nearly starts, Billy told Eifman about catching up with Kujo. Eifman remarked, "Oh really, with Ms. Kujo? Is she well?" Billy answered, "Yes." Eifman asked, "What about that incident?" Billy quoted Kujo, "I've already forgotten it,' she said." Just then, visual confirmations arrived about the Gundams. The Union officers were surprised as they have brand new equipment no present from their original profiles. As everyone watched on the battle, PMC and AEU forces have already lost about half their forces in merely two hours. As more data comes in about CB winning, Billy comments, "Jeez, isn't that a bit much?" Graham comments, "They're pretty overwhelming, those Gundams." Graham and Billy knew its a lost cause for the PMC/AEU counteroffensive, they headed to the lounge to talk and drink more coffee. Eifman came to them to say, "Seems it's over." Graham says, "It appears that the AEU's firearms have been outclassed." Billy says, "I wonder...they may have lost over 20 mobile suits. After this, I believe the AEU will follow the will of its people and be pushed further into an arms race. Give the loan Moralia received, it looks like there's more than just a cooperation with PMC." Eifman comments, "Sad, isn't it. No matter how showy the victory, Celestial Being is destined to be hated by the world." Graham asks, "Professor, do you think they're racing toward their own destruction?" Eifman answers, "Given their actions, I think they're seeking it. There aren't many of them, but that's how I see it."

GN Tech Investigation

After the battle, Billy and Eifman have since continued tuning Graham's Custom Flag and continue on their research into the mysteries of CB's GN-tech.

Project G

Billy wanted to see Kujo again, he sent her a message with sensitive military data to get her attention. Kujo met Billy at a Cafe Bar, surprised at Billy to give her such sensitive military information. He used the excuse of her tactical analytical expertise to talk to her about possible tactics and capabilities the Gundams might have. Kujo didn't feel entirely comfortable with their discussion and Billy backed off. Kujo had to leave because she had important matters to tend to. Billy felt content to risk national security files to see Kujo again. Little did he know he was tipping off the enemy.

Surprise Attack

The Throne Gundams began their first assault mission on Union, destroy MSWAD base. Prof Eifman and Billy were on the base, continuing their investigation on Celestial Being. While Billy was in the open, Gundam Throne Eins fired its GN Mega Launcher. The blast devastated the base and killed Prof Eifman, Billy was blown away by the shock force of the GN Beam and suffered a head injury and broken left arm.

Pressing On

After the Throne Gundam's killed Professor Eifman, Billy continued R&D/investigation over Celestial Being. Billy couldn't rest, knowing the death of Prof Eifman and continued work on MSWAD despite his injuries. While working on Graham's MS, he and Graham had a talk over the attack. Billy told Graham that Prof Eifman was close to a breakthrough in their investigation on Celestial Being and believes their intervention was no coincidence. Billy mentioned to Graham that Celestial Being was tipped off, Graham suspected a traitor, but Billy thinks otherwise since all the personnel were carefully selected. Billy continued researching the GN Tau Drives given by a benefactor inside Celestial Being. He helped modify Graham's Custom Flag into the GN Flag and made it space combat capable.


After the fade of Celestial Being, nothing was mentioned about Billy. It's only an assumption that he continued on GN Tech development for UN Forces and later the Earth Sphere Federation. Somewhere in between that time, he found Kujo again, lost and depressed.

Looking After Kujo & CB Revelations

Billy in the last 2 years was actively tending to (Sumeragi Lee Noriega) as she became a severe alcoholic drunk. While tending to her daily drinking habits, the door rang and Billy was surprised that Sumeragi has a friend visiting. Billy thought that inviting a guest to dinner would lift her spirits, but he was in a rude awakening when he opened the door. Setsuna quickly identified Kujo as Sumeragi Lee Noriega of Celestial Being. In the aftermath, he was left behind in the apartment in rage as the person he loves is affiliated with the infamous group, Celestial Being.

Anti Celestial Being Campaign II

Joining A-Laws

Billy felt all his feelings and efforts for Sumeragi was abused by her. With so much anger and discontent, he decided to join A-Laws as their leading mobile suit developer (with the rank of Technical Captain[1]) as his way of getting back at her, possibly believing she was responsible for Professor Eifman's death. He develops a new suit to bridge the distance of the combat capabilities of a Gundam, the Masurao. While developing the new unit, he found some of Prof Eifman's handwritten notes at his home. He paid close attention to Eifman's theories on the GN Drive and beta tested it on Masurao, a pseudo Trans-Am System, which imitated the enhanced abilities of the Gundams but the disadvantage was that the GN drive would be depleted and powerless. After he proved Eifman's theories to be correct, he gave the Union/Samurai-themed MS unit to Graham/Mr. Bushido. Billy believes Masurao will contribute to the end of Leesa and brings out a picture of him and Kujo from when they were back in the university and rips it to shreds before toss it into the air, saying, "Goodbye, Kujo."

Billy later attends another A-Laws onslaught against Ptolemaios, hoping to see Celestial Being finally be destroyed. However, they had to pull back when they receive word of a coup d'état conducted by the regular army.

Battle at Lagrange 2

Billy was present on board the Innovator mothership/mobile colony Celestial Being (mothership), which wiped out the A-Laws fleet while it was engaged against Celestial Being, Katharon and the ESF Coup d'état faction. He supervised the switching of the ship's GN drive Taus.

When Ptolemy 2 rammed into one of Celestial Being's docking ports, Billy led a squadron of automatons on board the ship. He came face-to-face with Sumeragi, when she ventured to deal with the intruders. They argued over their views of a world ruled by the Innovators. Billy believed that humanity is better off, locked in a cage, where it can't harm itself, while Sumeragi counters that humanity is better building its own future, even though there will be conflicts. He was close to shooting her when Setsuna/GN-0000+GNR-010 00 Raiser initiated Trans-Am Burst, which covered the colony in massive amounts of the GN particles. He begins hearing the thoughts of everyone around him. Telepathically, Sumeragi apologized for taking advantage of his feelings for her for her own gain and slowly approaches him. He weakly tries to defend that his actions were for peace but blurted out that he always loved her. Together, they fought off the automatons until GN-009 Seraphim Gundam used its TRIAL field to disable all units connected to Veda.

But as they were about to celebrate victory, Seraphim is damaged by Ribbons Almark/CB-0000G/C Reborns Gundam/Reborns Cannon . Ptolemy 2 goes to assist Setsuna/00 Raiser against Ribbons Almark/CB-0000G/C Reborns Gundam/Reborns Cannon. Ultimately, the former wins.

Peace & Moving On

Billy returned to mobile suit development when the A-Laws were exposed. However, his uncle committed suicide to escape prosecution.

ELS Conflict

Developing the Brave

Billy was shown to be aiding in the final assembly of the Braves in some ex-Union base. After this, he was transferred to the Celestial Being Mothership to aid in the investigation of ELS behavior along with his girlfriend Mina Carmine.

Investigating with Mina


Mina Carmine
She became Billy's girlfriend sometime between the Innovators defeat and the ELS conflict. Sometime after the ELS conflict they later married and had a son that later evolved into an Innovator.
Sumeragi Lee Noriega
Billy studied with Sumeragi under Ralph Eifman during their earlier years. He developed feelings for Sumeragi when they studied together and remained interested in her during their time apart. Unknown to Billy, he's been giving off tactical secrets from the Union to Sumeragi, Celestial Being's tactical analyst. After learning the truth behind Sumeragi's affiliation with Celestial Being, he focused all his negative feelings for her into his work to help bring down Celestial Being. Eventually, Billy is able to admit his true feelings for Sumeragi and joins with CB to bring down Innovators. It is unknown whether they are still close friends.
Ralph Eifman
Billy and Eifman both co-developed Union Flags and the leading authority on mobile suit engineering. They once worked with Sumeragi, but after a past incident she left and Eifman asks Billy of her from time-to-time. It's unclear whether or not Billy had received education by Eifman or was merely his assistant.
Graham Aker
Billy and Graham presumably met through the Union military. Graham had consulted Billy over fine tuning his mobile suits into machines such as the GN Flag, Masurao, and Susanowo.
Homer Katagiri
Billy's uncle and the leader of A-Laws. The two have a close relationship, though they do not share much time together.
Setsuna F. Seiei
Billy and Graham first met Setsuna in Azadistan, figuring that he was a local. Years later, Billy did not recognize Setsuna when he came to his door and exposed Sumeragi's identity.



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