Oh god, just this moment of empathy.
Rin has spent so many years feeling not good enough, feeling second best, feeling like he has failed his father, failed to be a good son, failed his own dreams because he is so torn up over his inferiority complex and losing to Haru all those years ago. And here is Nitori, able to verbalize those feelings more honestly and truthfully than Rin EVER could. And for the first time it’s like Rin can SEE him, really SEE him, because he realizes he’s looking in a mirror.
That moment when the cute side character becomes vital to one of the main characters.