The Classmate

Eunha's high-school life takes a dramatic turn when her bullies force her to perform a ghost-summoning ritual that brings Haesu, a charming yet unsettling spirit, into her life. Claiming to be her friend, Haesu starts exacting revenge on Eunha's tormentors. However, his violent methods concern Eunha, and she begins searching for answers to uncover the past and his true identity.

(Source: Webtoon Entertainment)