Only I Know the Ghoul Saved the World, Vol. 1 (light novel) — Myojin Katou

Not the hero they want, but the one they need!

A heavy mist has fallen over the world, twisting many humans into horrifying monsters. Humanity is losing the battle against these grotesque fiends, but it is not entirely without defenses. Leon Crossheart is an adventurer, a hero, a hunter of ghouls... and a ghoul himself ! His strength and regenerative ability are unmatched, but this power comes at a price: He's reviled by the very people he strives to protect. However, there is one girl he saved who sees beyond his frightening appearance. Alice is so grateful that she wants to be his disciple, and she isn't taking no for an answer! Will she bring light into his dark and bloodstained life, or will he keep her -- and therefore his own salvation -- out of reach?