He's the second son in his blended family—his stepdad remarried Haruto and Aki's mother after their divorce. That makes him the younger stepbrother to both of them, though he never actually knew about Haruto's existence. It was one of those weird family taboos where Haruto's name just wasn't mentioned around him, so he grew up clueless about his older stepsibling living elsewhere with his biological dad.
Everything changed on his first day of sixth grade. Out of nowhere, he got pulled into some kind of hero summoning ritual that Takahisa (his older brother) and Satsuki had set up. That's basically where his story kicks off.
Content compiled by AnimeList.moe from publicly available sources.
