Sei is a young boy whose family runs Iori Hobby Shop in a small town, which for yet to be explained reasons, located in a residential area. As such, the shop doubles as his family's home. His father was a Gunpla Battle champion many years ago, and Sei has inherited his skill at building and customizing Gunpla, but not the skill and sense in piloting one. Because of this, he has Reiji pilot his Gunpla while he takes charge of analyzing the opponent to provide navigational and tactical data during battle.
Over the course of the World Tournament, Sei matures not only as a builder but also as a pilot, now no longer a pilot that just about anyone can toy around and trash within seconds. While still nowhere as good as the likes of Reiji and Ricardo Fellini, he is now more than capable of hold his own against top class pilots, as seen in his battle with Reiji at the festival.
It was revealed by Reiji that it wasn't Sei's lack of piloting skill that got in the way of making him of good fighter, but his subconscious reluctance to damage a Gunpla instead, due to his love for Gunpla in general. Reiji further added that to be a good fighter, he needs to have the resolve to win regardless of the odds, including severe damage on the Gunplas from both sides. These words finally unshackled Sei's restraints and enabled him to use his piloting skills fully, allowing him to go head-on against top class pilots like Tatsuya Yuuki.
Reiji is a mysterious red-haired boy who befriends Sei at a park. He gives Sei a gem, telling him he will be at his side whenever he wishes. During a Gunpla Battle between Sei and Susumu Sazaki, Reiji suddenly appears and commandeers Sei's Gunpla, giving him a surprise victory. Reiji also shows adaptability with any Gunpla, as he can defeat veteran Gunpla Fighters with just an RB-79 Ball. Despite his natural piloting abilities, Reiji has no concept of everyday life such as school or commerce.
Following their battle with Tatsuya Yuuki, Reiji suddenly glows and vanishes in front of Sei. 
Reiji explains to Sei that he is a prince from a country called Arian in a distant planet; one day, he stumbled upon a treasure that enables him to travel to Earth and back. Despite panicking and demanding an explanation after seeing Reiji's stunt, Sei drops the subject entirely after finding Reiji's explanation absurd and he can never get a believable one.
The two have somewhat of a brotherly relationship; for example, arguing over having to repair the Star Build Strike despite Reiji's inexperience and reconciling after Sei was fascinated by Reiji's quick skill (with some help from Takeshi Iori) in building the Beginning Gundam. Sei later gave the gem from Reiji to Aila Jyrkiäinen through China, hoping that it could help patch up their relationship.
China is Sei's classmate and later girlfriend. She walked away when she saw Sei talking to Mihoshi inside the Hobby Shop. China later went and cheer for Sei in the 7th World Tournament. They later went to Paris for her to become an artist.
An Upperclassman in the Seiho Academy. He is considered the best Gunpla Fighter in school making him Sei and Reiji's rival. He seems to have taken an interest in both protagonists, to the extent of making the uninterested Reiji fight him to get his drive on. Despite being their rivals, Sei still shows great admiration and respect for Tatsuya's way of thinking towards Gunpla.
Susumu Sazaki is a local resident who is friends with Sei. He proposed for Sei to have him pilot the Build Strike Gundam, but Sei refused on the grounds that his piloting skills are extremely aggressive. Though he holds no hatred nor disgust towards Susumu, he doesn't like the way that he fights, which is overly aggressive, overly brutal and utterly inelegant.
Rinko is Sei's mother and the current owner of Iori Hobby Shop. Sei is sometimes annoyed by his mother's intrusiveness regarding his relationship with China, nonetheless, they care for each like any mother and son.
Takeshi is Sei's father, who placed runner-up at the second Gunpla Battle World Tournament many years ago. He left Japan to promote Gunpla Battle worldwide and entrusted his hobby shop to Rinko and Sei.
A 7th-grader attending Seiho Academy whose family owns the Iori Model shop. He has been a Gunpla fan from a young age and first watched Mobile Suit Gundam when he was three years old, unconsciously memorizing every line Amuro spoke in all 43 episodes. Sei has shown an advanced knowledge of Gundam and a proficiency with building Gunpla that is often compared to his father's, who was a remarkable Gunpla builder himself. For example, he gave an excellent overview of the XXXG-01W Wing Gundam's abilities and characteristics to a customer. (Sei also gets so excited that when talking about the Wing, he starts to briefly tell the customer about the history of the unit). Sei is very well known as being an excellent modeler, but in the same token was a poor Gunpla Fighter with a long record of washing out in the first round but as the series progresses his skill improved (this was proven when he was at even odds against Reiji in episode 23). Sei is the creator of a customized GAT-X105 Strike Gundam model, the GAT-X105B Build Strike Gundam: A model focusing on mobility, it specializes in quick, hit-and-run attacks.
Years later, Sei also constructed the BG-011B Build Burning Gundam, a melee combat model, to compete in the 11th World Tournament. After the tournament, he improved the suit and for unknown reasons hid it within a modified breakaway MS-09B Dom chassis (which was fully functional as a Dom until the outer layer was breached) and further hid the concealed Gunpla inside the base of his World Championship trophy, which he left in a display case at the Seiho Academy Gunpla Battle Club. It was revealed that Sei had left the Build Burning Gundam in the trophy to have it waiting for Reiji's return; Sei had specifically constructed the Build Burning Gundam to maximize Reiji's fighting potential. After watching Sekai Kamiki battling with the Build Burning Gundam as part of Team Try Fighters, Sei made the decision to give the Build Burning Gundam to Sekai because his fighting style reminded Sei of Reiji.
- As an operator, Sei is very mindful of his situation and the status of both his opponent's and his and Reiji's Gunplas. His tactical skill and use of his surroundings is quite similar to Mobile Suit Gundam's Bright Noa, an experienced officer and extraordinary tactician.
- At the start of episode 1 and during the imaginary battle with Mao, Sei is shown to wear Kira Yamato's pilot suit as well as based his Gunpla after the GAT X-105 Strike Gundam; indicating that his favorite series among the franchise is Gundam SEED. This may also reference to SEED's popularity in Japan.
- His choice of Gunpla, the Build Strike Gundam, might be an allusion to the fact that his mother is voiced by Kotono Mitsuishi, the voice behind Murrue Ramius from Mobile Suit Gundam SEED.
- It is revealed in Episode 23 and Episode 25 that Sei has piloting skills that are on par with Reiji's and Yuuki's but didn't have self confidence at first due to his reluctance to have his Gunpla damaged and damaging another person's Gunpla.
- According to Episode 15, his piloting stats are as follow;
- Attack accuracy: 26%
- Attack evasion: 9%
- Attack blocking: 69%
- Ground battle rating: C
- Aerial/Space battle rating: B
- ↑ Gundam Build Fighters Episode 1, Sei and Reiji
- ↑ Gundam Build Fighters Episode 2, The Crimson Comet
- ↑ Gundam Build Fighters Episode 3, Full Package
- ↑ Gundam Build Fighters Episode 4, Gunpla Idol Kirara ⭐️
- ↑ Gundam Build Fighters Episode 20, Aila's Betrayal
- ↑ Gundam Build Fighters Try Episode 24, Final Burst