[Kor wants to object to Vanitas' claims about Nobodies, seeing as how one of his bondmates is a Nobody, but he bites his tongue and abstains from snapping back at him for that. This isn't about Roxas, this is about Sora.

So he listens to Vanitas' explanation, and as he does so, his mixed feelings he's held towards Vanitas suddenly makes a lot more sense to him. Why he gravitated towards Vanitas even though he was quite frankly really hard to actually get along with. It wasn't just because of his connection to Sora.

His story reminds him of his own situation with Creed. Their circumstances were a little different; Vanitas was literally half a person, while Kor had been a host to Creed, but he could understand and sympathize with that feeling of incompleteness, of wanting to repair what once was. Even now he fights hard to try and repair the holes left in his Spiria by Creed's escape from it...]

Sorry. [Kor shakes his head.] But I can't do that. Not if Sora asks me to help him again. [Kor's already in this deep and has already promised him he'd see it through.]

But I'll take responsibility for my actions. So instead, I'll just have to do the same for you too, make sure your heart is healing the way you want it to be.