He is one of the two main protagonists of Gundam Build Fighters. Although his skills in Gunpla building are exceptional, Sei's Gunpla Battle piloting skills were lacking until they gradually improved over the course of the series to the point where he was able to hold his own against Reiji. He is the builder of the GAT-X105B Build Strike Gundam and the SB-011 Star Burning Gundam. 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. China Kousaka 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.