Finally, the Gold End (kind of sort of) explains things. Chihiro wasn’t loved enough as a child or something and everyone eventually left him (probably because he was so clingy). However, Kanade manages to calm him down by telling him that his self sacrificing obsessiveness doesn’t make her happy since it’s ignoring how SHE feels. But she’s also says that she’s not going to abandon him or anything, and all of this gets him back to the person that he was at the beginning of the route. Skip to graduation and Kanade STILL calls Chihiro “sensei”. Though I guess he still is one. He’s an author now, but somehow he went from children’s writer to a guy who writes porn based on him and Kanade’s bedroom antics. At least it’s not porn for children. Probably. Hopefully.
…I give up on this character. Let’s see how KuroKin butchers the rest of these routes for me.