Ysley is an Idaten born roughly a century ago from a natural outbreak—pretty rare for his kind. He's basically the intellectual of the group, way more interested in books and research than actually fighting. Since he came into existence naturally rather than being created, he's spent a lot of time trying to figure out who he actually is through study and investigation.
He gave Rin's training a shot early on, but bailed after just a couple months. The guy wasn't exaggerating when he called it brutal—it was legitimately torture for him. His real passion has always been digging into human history and exploring ancient ruins, which tells you a lot about where his head's at.
Things got serious when a magical being showed up and basically wrecked him, putting a hole straight through his chest. Hayato had to step in and finish the fight. That reality check made Ysley realize the demon threat was only going to get worse, so he decided to bite the bullet and get back into training with Rin, this time for real.
Content compiled by AnimeList.moe from publicly available sources.
