The Lost. "... I need to go. Now." "I can't let them push me down again, I can't let anyone else get hurt-" "...it's gone too far. Oh god, it's gone too far..." "I can't return from this." The remnant consciousness and conscience of what he once used to be. He hates what they've done... What he's done. Because he knows they are as much a part of him as he himself is. "The only way to atone for what I've done is to isolate myself." "I'm so sorry, Kacchan. I can't be near you." Izuku Midoriya's mind has fractured. For The Lost, existence is a living hell. He is the remnant conscience of the boy the minds used to be, before Izuku's mind broke into three people. The other two force him out of control of his own body, keep him down, shove his consciousness into the void where he falls and falls and falls and falls and f a l l s The Lost is the only one who can be near her and still retain a measure of his sanity. It's too bad he's never in control when she's around.
If you wish for more information, simply ask. Art by me (using Ibispaint on my silly little tablet)