The son of a legendary builder and fighter, Sei inherited his father Takeshi's love for Gunpla (Gundam models). He himself grew up to become a talented builder, but he struggles with fighting in Gunpla Battles, which has kept him from competing. He is 13 years old with blue eyes and blue hair, the latter of which he inherited from his mother Rinko. Whenever he isn't in school, he can be seen wearing an orange shirt under a sky blue hoodie with crop pants.
He spends most of his time building Gunpla and helping out his mother Rinko run their Gunpla shop in the suburbs. The less-than-ideal location means they don't get a lot of customers, yet that was his father's decision. His dedication to Gunpla is such that he would lock himself in his room to work on a project for long periods of time, absorbed in his craft. What comes out of it is rivaled only by the works of other high-level Gunpla builders.
As the builder and support player of the tandem with Reiji, he mostly provides intel and additional information about their Gunpla and the opponent. His knowledge is very deep and broad due to having been obsessed with Gundam since he was a kid. He would passionately explain every detail about every Gunpla to customers in the shop, even to the point of information overload due to his enthusiasm.
Upon gaining Reiji as his pilot, his confidence grows and he is able to fully unleash his imagination and technical brilliance with his Gunpla. He's always looking for improvements and additions to his builds, especially after each significant battle. His friendship and partnership with Reiji grows to the point of being able to discern what his partner is thinking.
While very skilled, he's not above recognizing others' abilities. When he and the Gundam Shingyo disciple Mao Yasaka showed their best Gunpla to each other, he immediately recognized how strong the Gundam X Maoh was. But while he may seem overwhelmed at first, he would then take it upon himself to find a way for him and Reiji to win. He compliments Reiji's prowess with his thoughtful design. While the samples he builds for the shop are pristine, Sei's custom Gunpla combine both craftsmanship with imagination.
Later in the series, Reiji tells Sei that his dismal piloting isn't because of his lack of skill, but his great love for Gunpla hindering him from having his own get damaged in battle. This realization helps Sei gain resolve to win against all odds and finally become a pilot who can hold his own against his partner and other top-tier pilots.
He also has a budding relationship with China, although both are still in denial of their infatuation with each other. It's something his mother pushes for in various ways throughout the series as she sees China as someone who is good for her son. Despite denying their romantic inclinations, he does care deeply for China and would enthusiastically help her, especially with building her own Gunpla.
Reiji is a mysterious kid who befriended Sei at a park one day. He then gives him a gem with a guarantee that he'll be at his side when he needs him. This promise was fulfilled when Sei was losing a Gunpla Battle against Susumu Sazaki as Reiji suddenly appeared, took the controls, and pulled off a comeback win. From then on, a close partnership formed and Reiji became Sei's Gunpla pilot.
Not much is known about Reiji other than his appearance, headstrong personality, and Gunpla piloting ability. He is a red-headed 14 year old who typically wears sunglasses on his head, a dark red vest over a yellow shirt, white pants, and black shoes. Other than that, people who get to know him eventually discern his brashness, love for food, and apparent lack of knowledge in common courtesy.
Perhaps his lack of common sense may not intentional at all as there are indications that he may be from another world, thus his misunderstanding of Earthly manners. His mysterious appearance, his apparently innate talent for piloting despite lack of knowledge in Gunpla, and the gem he gave to Sei give clues to his true origins, wherever that may be. He does tell Sei and Rinko of him being from another world, although he doesn't go into detail.
Later on as Reiji and Sei progressed through the 7th Gunpla Battle World Championship, someone did recognize the Gunpla pilot. Chairman Mashita, a petty thief in Arian who then became the CEO of Plavsky Particle System Engineering (PPSE) by stealing an Arista and creating Plavsky Particles for Gunpla Battle out of it, saw his Arista Bracelet and realized that he is Aria von Reiji Asuna, the Crown Prince of Arian. He saw Reiji's presence as a threat to his station on Earth and sought to eliminate him from the tournament by any means.
However, this turns out to be harder than expected as Reiji is no ordinary child. His piloting skills were such that Mr. Ral compared him to a Newtype when he first saw him fight. Moreover, he has the support and the customized Gunpla of Sei Iori, a talented Gunpla builder who fully entrusts Reiji with his creations. With both his growing ability and Sei's ever-improving Gunpla, their team has become a top contender in the tournament.
He is driven by competition and rivalry, and he will go to great lengths to become stronger. He fought 200 straight battles against the Italian champion Ricardo Fellini, which helped him tremendously. He would seem to get stronger after each battle, owing to both his talent and his will to win. While he could pilot any Gunpla off the shelf to great effect, he does his very best with Sei's Gunpla.
While he's perceptive as a pilot, he's dense with other things. While he had become close to Aila Jyrkianen, the Finnish pilot of Team Nemesis, he never realizes that she may have actually started to fall in love with him. When he inevitably got transported back to Arian in the end, she tagged along by making a wish to the Arista gem he had given her. Despite this show of devotion, he remained clueless and she was obviously vexed by it.
Sei took much from his mother Rinko, including her blue hair and cheerful disposition. While she may not be that familiar with Gunpla, having to defer to her son when it comes to technical details, she keeps the Iori Hobby Shop running even when her husband is away. Despite all of that going on, she seems to have done a great job at raising her son.
She can be somewhat intrusive, mostly due to her wanting her son to end up with China. Playing matchmaker whenever the two are in the same proximity, she eavesdrops and ogles with stars in her eyes as she imagines them getting together. But even though she can be rather pushy at times, she is sweet and very supportive of Reiji, her son, and anyone who she sees as a friend.
Aside from acting as a chaperone for the kids most of the time, she is also a pretty good cook, especially with her signature vegetable stir-fry. Like China, her support is unending and keeps Sei and Reiji's spirits up throughout the series. As one of the two adults in the group, she both maintains the morale and take care of their well-being so they can compete and do well.
She is also willing to help people however she can, like taking Aila in after learning of her situation. While Takeshi may not be present at the moment, she is still pretty much in love with him and remains loyal to her family. She's pretty much a supporting character at every sense of the word.
China is Sei's classmate and good friend who also seems to have a liking for him. She was not really into Gunpla at first, but her constant support for Sei leads to her wanting to get an understanding of it herself. While she may not know much about Gunpla, she is a painter who has won a number of competitions, and this translates well into her Gunpla painting.
She finds pretending and being insincere rather difficult, so much that she can't even put up a courtesy smile while helping out as a server in her family's restaurant. But because of this, she's trustworthy for her intentions and actions can be seen as pure and genuine. Despite being reserved, she can jump into action when absolutely necessary, and her piloting skills aren't bad either.
Her most notable attribute is her unending support for Sei throughout the series. In the epilogue, she does become his girlfriend and they travel the world together to promote Gunpla. She also has a little brother named Yuuma, who is a main character in the sequel Gundam Build Fighters Try, which is set 7 years later.
A 35-year-old mustachioed gentleman with an affinity for Gunpla, Mr. Ral is a neighborhood friend of the Ioris and regular customer of Iori Hobby Shop. He is also referred to as "Lieutenant Ral" and "Blue Giant," which are based from the Mobile Suit Gundam character Ramba Ral. He even uses the same mobile suit, the MS-07B Gouf.
He is a skilled and knowledgeable Gunpla builder and fighter with many years of experience and lots of connections in the Gunpla world. He is crucial to the development of both Sei and Reiji, giving them advice and leading them along their paths to improvement. Those who know of him and his reputation seem to show a great deal of respect, owing to an apparent glorious past in Gunpla Battle.
But it's not just his knowledge and experience that help, but also his lack of hesitation in springing into action when necessary. When he spotted a saboteur sneaking around behind the scenes in the tournament, he quickly shadowed the culprit and stopped him. No matter what the situation is, his support to the main characters throughout the series becomes invaluable.
Other than his love for Gunpla, he also seems to have a crush on Rinko and tends to get an itchy behind at the sight of blossoming youth in love. It doesn't seem like he's married or in a relationship, at least from how he's depicted in the series. He continues to mentor aspiring Gunpla builders and fighters after the series, including in Gundam Build Fighters Try.
During the start of the series, he is the president of both the student council and the Gunpla modeling club. He was also the protagonist of the manga Gundam Build Fighters Amazing, which traces his origins as the scion of a rich family who was committed entirely to his studies until he was introduced to the world of Gunpla Battles at 7 by a kid of the same age.
He loves building Gunpla and a good fight, and he got really good at both. For a while, he hadn't really faced a tough challenge, but then he met Sei and Reiji in battle and they soon became rivals. But before they could have a rematch in the qualifiers for the World Championship, he suddenly took a leave of absence from school and seemingly left them in the dark.
It turns out that it was because he became the third Meijin Kawaguchi, a title given to the most talented builder and fighter who embodied a certain high standard. The second Meijin had been a rather unpleasant competitor who only valued winning at all costs, but Tatsuya sought to change that perception of the role by going with the ideals of a true warrior who lived for an honorable battle instead of just victory.
Under the employ of PPSE, he has pretty much all the resources and knowledge on hand to potentially win the tournament, but he understands that it will take more than mere strength to win. Despite becoming Meijin Kawaguchi III, he hasn't lost the values he cherished as regular old Tatsuya Yuuki.
A warrior in battle and a playboy outside of it, Fellini is both the Italian Gunpla champion and a regular Casanova. Calling himself "The Italian Dandy," he carries his own brand of panache both in and out of Gunpla Battle. Everything about him is all about hitting it with the ladies, and it does earn him quite a few enemies along the way. When they do come for him, he fends them off with his Wing Gundam Fenice.
While he may be flirtatious and seemingly not serious, he's a formidable Gunpla fighter who would rather blow himself up than accept defeat. In terms of fighting style, he's well-balanced. He may prefer using his beam rifle to win, but he isn't adverse to drawing out his beam saber when the fight gets to close quarters. As a Gunpla Battle veteran, he can blow most opposition away and give high-level pilots a lot of difficulty.
He also doesn't seem adverse to helping other Gunpla fighters out. When he met Reiji for the first time, he met the red-headed youngster's challenge head-on and ended up fighting 200 straight battles with him. The experience greatly helped Reiji as he became much stronger after their "training session," blowing the competition away despite having his hands in bandages due to the strain.
Despite his waywardness, he does somehow find a love interest in Mihoshi, who's also known as the popular Gunpla idol Kirara. Somehow, she was charmed by him, even when he got drunk and started gushing over Gundam Blu-Rays.
He may dress like a country bumpkin, but Mao is no joke in Gunpla. While Sei Iori is the best builder of his age in the east of Japan, Mao Yasaka is the same for the west of Japan. A pupil of the Gunpla Shingyo School under the tutelage of Master Chinan, he's both talented and dedicated to Gunpla, making him a top contender in the World Championship. His Gundam X Maoh was acknowledged by Sei as strong, as Mao did with Sei's Build Strike.
But while Sei is a bit more reserved, Mao is more confident with himself. His building skills are undeniably good, and piloting skills are on par as well. He can get rather creative in battle as well, able to use unconventional tools and tactics to win battles. He prefers to fight from range whenever he can, but he isn't adverse to melee combat when there's no way to avoid it.
Having been a top pupil in the Gunpla Shingyo School, he was quite full of himself at first. Seeing this arrogance as a weakness, Master Chinan decided it best that Mao see more of the world. This leads Mao to cross paths with the gifted son of the only man his master had ever feared. Mao needed a rival to spur on his progress, and he indeed got one in Sei.
Despite being full of himself, he's rather weak when it comes to matters of the heart. He tends to get excited when he's with a girl he likes and moves too quickly, getting hurt when he's rejected and wailing about it incessantly to others.
Having been an orphan in the cold streets of Finland, she was taken in by the Flana Institute after her abilities were assessed. While she has neither building experience nor background on Gundam, she was trained to be a fearsome Gunpla pilot, one that would become a major threat in the 7th Gunpla Battle World Championship.
Despite being so skilled, she dislikes Gunpla Battle due to the Embody System, which increases her piloting abilities at the cost of her losing control of herself. She would lament on how everyone she faced would lose to her, which compounds her lonely existence, but she continues to fight because to her, the fear of being thrown back to the streets is the scariest thing imaginable.
Due to her poor background, she has an above-average appetite for food, which led to her running into Reiji. After a rough start, they became friends and Aila took a liking to him, although still hiding her status as a pilot. They spent time together during downtime in the tournament, including an impromptu session of Gunpla building with an overly-passionate teacher.
When they did fight, Reiji gave her hope in being freed and having somewhere to go home to. Upon shaking off the clutches of the Embody System, she became more verbose and animated, finally free to express herself and being able to finally love Gunpla Battle.
When Reiji inevitably returned to his world, Aila followed him by wishing upon the Arista that China gave her to stay with him, even if he was oblivious to it.
He may not be present in the series until the later episodes, but his influence pervades with various characters in the series. Takeshi Iori is apparently seen as a legend in the Gunpla world, being a former Gunpla world champion. He then most likely settled down with Rinko and became the owner of the Iori Hobby Shop.
Despite being a family man, he does seem to still move around as he goes missing during most of the series. It turns out that he's an International Gunpla Battle Official Referee, overseeing Gunpla Battles all around the world and catching those who violate rules and conduct unfair actions during play. He's quite enthusiastic in this role too, although it may be a bit too much as he does leave home for it in fairly long intervals.
However, he does make his presence felt, especially when he (forcibly) taught Reiji and Aila how to build Gunpla while in disguise. He then went back home to surprise his wife, but no one was home since Rinko had gone to the 7th Gunpla Battle World Championship to cheer for their son. After much wandering around without a clue, he realizes this and hitchhikes his way there to reunite with them.
Much of his talent and love for Gunpla had been inherited by his son, who had become a known figure in Gunpla on his own right. While he may have been physically absent for most of Sei's life, his son still took much from him, including his incredible love for Gunpla.