|Family||Sora Hashiba (derivative)|
|Relationships||Sora Hashiba (lover; depending the player choices)|
Sei and Sora don't hit it off immediately; at first the two argue, and then protest when Gaku and Sunao laugh at them for being so alike. After Sora saves Sei and Fuuta from three bullies, Sora thanks Sei for trying to help him during the fight, and asks if he's hurt. Sei starts to cry (much to Sora's surprise), and says that he was really scared. Sora comforts him, and Sei asks if Sora will protect him next time as well, to which Sora replies 'Sure. I promise, so don't cry'. Sora then goes on to say that he didn't have any parents, and he felt like he had a younger brother when he met him. Sei felt the same way, and from then on the two were close.
In the game, Sei is a clone of Sora, and first trusts Sora and Sunao.
- He is sometimes seen as Sora and Sunao's child.