So I saw this Kamui icon with him saying "gomen" or "I'm sorry" and all of a sudden this popped into my head. I just love it so much, the way Kamui looked, slightly red and looking so sad, and yeah... I could see him apologizing to Kotori, for whatever reason.
I'm not quite sure myself what he is apologizing for. But to me, maybe he's apologizing for acting so cold and mean towards her when he first moved back to Tokyo? ^^ It's up to your imagination~
(Yes, this has absolutely no plot)

"... Gomen."

The way his eyes, his face, his whole expression, looked like when he said that one single word above a whisper, made Kotori's heart ache.

It wasn't a quick "yeah, sorry" type of apology, where he obviously wasn't sorry at all, not even looking into her eyes.

It wasn't a harsh "sorry" type of apology, where he said it in a brusque manner.

And it wasn't a nonchalant "uh, yeah, sorry" type of apology where he was completely indifferent on the matter, and was apologizing just because he felt like he had to.

No, this soft-spoken "I'm sorry" was sincere. It was heartfelt. It was meaningful.

Because Kotori could see that Kamui truly was sorry to her.

And in reply, she could only give him a bright, gentle smile, and reassure him that it was "All right".