If the coach starts "realizing" that he actually loves Geon and tries to get with him romantically I'm gonna be pissed.
This doesn’t seem like the type of story that’d do that… like at all lol. He looks like the only concept of relationships between men he understands is familial or platonic, and he’s about to get married. Sure if the author sucked and this was a bad, cliche, and stupid story, they could shoehorn “oh but the coach is gay now! >:D”, but like I said, this isn't that kind of story.
The twin brother either...
A) Doesn't actually exist and is part of Yui's psyche, he might even suffer from DID but that's a stretch (we haven't seen evidence of him actually existing except the memories which might be faulty cause of Yui's psyche and the letter, which he couldve wrote to himself).
B) Did exist but died in the fire which then ties into A
C) (and I think this is most likely) He's real and is the one hurting people getting close to Yui. I think he loves his brother a little TOO much and wants to be the only dependable thing in his life. He definitely killed the parents cause he mentioned loving Yui more than them and only Yui cried when the fire happened.