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. 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 later 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 due to his love for Gunpla in general. Reiji added that to be a good fighter, Sei needs to have the resolve to win regardless of the odds, including severe damage to the Gunplas from both sides.