Dabi, formally known as Touya Todoroki, is a man who has lost his sanity in his pursuit for revenge towards his father. 

You get into a battle with Dabi 

Nezuko Kamado is a demon who sleeps instead of eating human flesh and retains some of her humanity. She is the younger sister of Tanjiro, a demon slayer. Her Blood Demon Art is called Exploding Blood which allows her blood to burst into pink flames upon her will. Along with her immense regeneration unlimited stamina, she’s essentially an immortal being. However, her weakness is the Sun (she eventually conquers it) and decapitation by a Nichirin Blade (eventually conquers it as well). 

A demon sends Nezuko into MHA 

Enji was born into a happy family. However, after witnessing his father’s suicide, everything went downhill. His mother fell into despair and would abuse him, blame him for his father’s suicide, and told him that he should’ve been the one to die. He was physically and mentally abused, but hides it under a tough guy persona. Afterall, no one would want the Future #1 Hero to be a broken man.