Who is it?

I think it might be Boone. I've seen him get massively crushed and overwhelmed and all that happens is he goes unconscious until I kill everything else then he gets up again.

Companions are invulnerable in casual mode. I don't think you can kill kids.

It's Victor. You can't kill him because he's every Securitybot. You kill one, another will take its place.

I killed him and he hasn't come back, he attacked me when I killed Mr. house.

Victor died while following me outside the first town last night (he and geckos killed each other). For other reasons I used an old save and still have him alive as of now.

He may be killable later, but he's unkillable early.

Actually, its Yes Man, at least after you Not sure what happens if you try before that, though.