Nightmare Eater

Tsukuru Kanomiya has had a special ability since childhood: he can see the manifestation of someone's misfortune in the form of a black mist, and he can even purify them. Kanomiya has spent most of his life keeping his ability a secret, but on the day of his high school entrance ceremony, he sees a beautiful girl, Haru Arisu, emitting a special kind of black mist. Smitten with her beauty, and worried for her future, Kanomiya stalks Arisu...! A beautiful exorcist and a wimpy sidekick, an action-packed supernatural school adventure begins!

(Source: Kadokawa, translated)