Spoiler — from the ending. Tap to reveal.
“I'm me. I'm me.”
2 quotes from My Hero Academia, each with commentary on what it means and why it stuck.
Cannot verify his own primacy and never will. The horror is administrative rather than physical, and the thing that keeps him functional is being told who he is by people with no reason to lie.
“I'm me. I'm me.”
“I wandered around, searching for myself. And I found myself, blessed with friends better than I could ever be.”
Read literally, which the series invites, searching for myself is not a figure of speech in his case — he has produced copies and lost track of which one he is. Finding himself therefore means something no other character means by it. He did not resolve the question; he was assigned an answer by people who treated it as settled, and the assignment held. That is what he is calling a blessing. It is also, unmistakably, a criminal organisation, and the line sits there daring you to say that the only functional support structure available to him was the League of Villains. The story does not soften this. Nobody else offered.
Repetition as load-bearing structure. He is not making a claim, he is running a check, and the check has to be re-run because the answer will not stay put — a quirk that produces copies of yourself, used once too often, leaving a man who cannot establish whether he is the original. The horror is administrative rather than physical. Nothing hurts; he simply cannot verify his own primacy and never will. What the series does with him is unexpectedly gentle. The villains, who are a criminal organisation and are not sentimental about much, treat the question as settled on his behalf and address him as though it were never open. That is presented, without comment, as the thing that keeps him functional. Being told who you are by people who have no reason to lie turns out to substitute adequately for knowing.