I'd be pretty hard for him to even do by mistake, considering how much longer he's known Kai compared to anyone significant who could show up and how much time and emotional investment he's paid into him as a result. This... this is something he absolutely wouldn't want to lose. He'd never be able to forgive himself if he did, and if Kai got hurt as a result, he's sure.

"Hmm," he hums softly, letting his head come to a rest at Kai's shoulder. Even if Shinji knows that this is so much more than he deserves, that he'd want Kai to find happiness with someone actually good, it would be really, really hard for him to let go at this point. How truly stupid of him.

"I'm not going anywhere," he says, a bold little bubble of honesty surfacing with the encouragement of the hot springs. "However long you'll have me, I'll be right here."