Billy Katagiri

He's an engineer and technical adviser for the Union's MSWΛD/Anti-Gundam Investigative Squad "Over Flags" and the co-developer of the SVMS-01 Union Flag. He's a good friend of Graham, former colleague to Sumeragi, and assistant developer with Professor Leif Eifman.

Personality
Billy is a highly skilled scientist of multiple disciplines for mobile suit development. He's overall a sentimental, passionate, and caring person for those he likes. After learning Sumeragi is part of Celestial Being, his emotions had taken a negative change. He's now spiteful, vengeful, and discontent how things turned out between him and Sumeragi. He has since poured all his feelings into mobile suit development to help cope with his anger/hatred against Celestial Being.

Graduate School
Before Billy's days as a Union Engineer, he was a college student studying with Kujoh (Sumeragi) and Professor Eifman. The three were involved in government military projects, but the depth of their work was never explained. Their work resulted in Kujoh resigning, but Billy and Prof Eifman continued R&D for Union Forces.

Anti-Gundam Coalition
Since the first Gundam attack at AEU, Union immediately started an anti-Gundam coalition. Billy, Leif Eifman and Graham were the first to be reassigned to the group because of their qualifications. Billy and Prof Eifman worked side-by-side, using all their engineering and scientific experience to learn the secrets behind the Gundams power source and capabilities. Billy and Eifman reached to similar theories about the GN Drive, but couldn't replicate one as they are 200 yrs behind in such technology.

Chasing Kujoh
Throughout the exploits between the world and Celestial Being, Billy thought about Kujoh. Billy haven't seen Kujoh in years and wanted to catch up with her.

Reunion
Billy and Kujoh meet up at a bar in Union territory to catch up. Billy wasn't shy to show his feelings for her and wondered about her feelings about that incident since they last saw each other. While Billy was glad to hear, I've already forgotten about it, but didn't look assured from her response either. Their evening had no closure as both had priorities in the following day.

Operation on Taklamakan Desert
Billy wanted to see Kujoh again, he sent her a message with sensitive military data to get her attention. Kujoh 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. Kujoh didn't feel entirely comfortable with their discussion and Billy backed off. Kujoh had to leave because she had important matters to tend to. Billy felt content to risk national security files to see Kujoh 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 (Mobile Suit Weapons & Development) base. Prof Eifman and Billy were on the base, continuing their development of the OverFlags and 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 OverFlag into a GN Flag and made it space combat capable.

Aftermath
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 Kujoh again, lost and depressed.

Revelations
Billy in the last 2 yrs have been actively tending to Kujoh (Sumeragi Lee Noriega) as she become a severe alcoholic drunk. While tending to her daily drinking habits, a door rings 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 Kujoh 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.

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 as his way of getting back at her, possibly believing she was responsible for Professor Eifman's death. He develops a new suit, the Masurao, to mimic 00, installed with an unknown system which he found in Eifman's handwritten notes, which was passed onto Graham, who now calls himself Mr. Bushido. Billy then 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."

Sumeragi Lee Noriega
Billy studied with Sumeragi under Leif 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.

Leif 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 Aifan asks Billy of her from time-to-time. It's unclear was Billy educated by Eifman or merely his assistant.