I also kind of theorize that Isa/Shuu is more likely to yield to people who are relentlessly nice to him, since that is something that Ryuuji and Hiyoko (on Shuu Route) have in common. Though I should probably play the game more to support that.

Isn't it just? It makes you wonder just how much Ryuuji picked up on (whether he was oblivious and unintentionally feeding the crush, or realized full well that's how Isa felt and kept him at arm's length but did a terrible job of it, or... even something more reciprocal, which is a little creepy given the age difference, BUT THIS CANON IS ALL ABOUT THE CREEPY). So many options to contemplate.