[Katsuki winces, when he's yelled at. Not surprised that Izuku, fresh from his little self-sacrificing journey, would be so angry after finding out that Katsuki's injury was given to him by the Villain Izuku was trying so hard to protect him from. He knew he wouldn't take the news well.. but what else was he supposed to do? Lie to Izuku? Tell him his insane arm injury was from some stupid Quest?

No, he couldn't do that. Izuku knew him too well anyway. He'd be able to tell immediately that Katsuki wasn't telling the truth.]

...technically it was Shigaraki. I think.

[Does he want to get into the whole "they merged into one guy but also weren't merged at the same time but he's pretty sure All For One was in control" thing? It's not even something Katsuki really understands, himself...]

But the Villains were smart. They separated the two of us, because they knew they had no chance if we were fighting together. And "Shigaraki" tried to kill me before you could get to us.

[Katsuki's speaking calmly as he explains himself to an angry, shaking Izuku. Wishing there was any better way to break this news, or comfort the other boy in knowing that it wasn't his fault. But he knows Izuku's blaming himself for this. And he knows he'll continue to blame himself, even after Katsuki tells him everything.]

I survived. I'm alive. But he crushed my arm... completely. And whatever magic they used to fix it here didn't do the job. It wouldn't move.

[He doesn't want to talk about his heart. He's not going to. It's enough as it is that his arm was destroyed-- Izuku doesn't need to know about just how close Katsuki really was to dying.]

I had it removed a month ago. I'm replacing it.