Bokaksaeng

In middle school, Cheol Im was a living legend for his fighting prowess. Years later, he learns that his childhood friend from the orphanage, Sol Kim, was bullied to death. Determined to get revenge, Cheol returns to school. But his plans take an unexpected turn when Ha-yun Seo, a fellow orphan who was like a sister to him, is assigned as a teacher at that very same school. Now, Cheol is forced to choose between his thirst for revenge and his loyalty to the person who once protected him.

(Source: WEBTOON)