It's a luxury to be comfortable enough with someone to tease them in a harmless way, like a kitten batting another with sheathed claws. Midoriya never had that as a child, but he's learned it since.

"I don't wake up Kaworu-kun unless there's something he'd want more than sleeping, and I shower." See, Paul, simple solution, is the tone and smile not spelled out in words. "If you don't take care of yourself, we can't spend time sharing notes." A nerd knows how to threaten a nerd. He does not say he knows what sleeplessness looks like.

He turns his head towards the other two so Paul won't run afoul of the horns curling over the sides of his head. With most of his face buried in his hair, he presses a small kiss to his head. He does not give Paul a coveted, highly precious cheese cracker. He runs his fingers through Kaworu's hair near his horns instead, a subconscious reminder for him to be careful when roughhousing.