Ken growls in breathless satisfaction at the sound from Chikusa, and it only makes him redouble his efforts. They'll both be sore later, they always are, but it's always so worth it. Ken thinks so, anyway, and obviously Chikusa agrees because he likes this, he lets Ken keep doing it. That's always a little point of pride, for Ken. He's no Mukuro, but Chikusa likes him enough to have sex with him anyway.

It doesn't occur to him that this logic works in reverse, but then it doesn't occur to him that Chikusa would ever need evidence of how Ken feels about him. Granted, it's been a long time since Ken was emotionally straightforward when it comes to softer emotions like caring about people, but Chikusa always calls him out on things like that, so obviously Chikusa must know.

Ken's more ferocity than stamina, focuses on the intensity of the experience both because that's what naturally appeals to him and because it gets the most powerful reactions from Chikusa. So when Chikusa gets even tighter around him than he already was, that's already the beginning of the end. Ken's teeth find the back of Chikusa's neck and he bites as he keeps thrusting, teeth digging in harder and harder until, by the time his hips slam him deep into Chikusa one last time as his orgasm hits, he's tasting blood on his tongue. He's not aware of the sound he makes when he climaxes, but it's utterly feral. If he wasn't preoccupied with biting Chikusa, it'd probably be a roar.

He's too quivery and disoriented in the aftermath to be aware, at first, if Chikusa came with him or not; his hand offers a few more rough strokes to his cock, just in case.