i wonder how the story would have gone if Kou-kun didn't run away
If Kou didn't disappear back then, maybe in turn, it wouldn't have caused Shion's bullies to stop bullying him until he graduated from high school. That in itself is one problem, but at least Kou is still there as Shion's (debatably healthy) mental and emotional support... Right?
I don't think Kou would've acted on his feelings if he didn't run away, but bearing the guilt of continuously having those sorts of feelings while hiding them
would have affected him mentally, and maybe eventually make him snap and cause a fight/misunderstanding between the two... And who knows what would've happened to Shion if he believed that it's his fault, maybe he would've spiralled even worse and had heavier feelings of hatred if they ended up growing apart like that anyway (with more bad feelings this time)...
This would arguably be an even worse scenario than what happened... Is what I think. (Maybe its just my angst-loving brain though)