[Tenses up when Keith stumbles back and he's quick to lurch back again, kind of curling into himself-- Keith's reaction to him more upsetting to him than the words themselves, really. He can recognize that urge to bolt in Keith and that's-- upsetting, to know he's the cause of it.]
I'm sorry. [An automatic response-- please, please don't run away from him. Not yet. Not when he doesn't know if they'll have any other chance to be here like this again after this.]
... I'm your friend. [He almost says it like a question but pushes past that urge, saying it more confidently than maybe he feels.] You're my best friend.