I'm me. [ 僕は僕だ it's an epithet he's repeating for himself as much as he is Marona. He curls a little tighter and stills. ]

I'm not worthy of being called that. And I'm not your big brother. [ He laughs, swallowing his tears and turning them into an emotion he does understand, smothering everything else. ]

...but it doesn't matter, do what you want. [ It's the best he can do when he's this broken mess of a person, but it might mean something in that he hasn't pulled away. And he hasn't actually denied her properly either. ]