Fall Madly in Love and Sing: R. I. P.

KAI is a gang leader known for his rapping skills and less-than-pleasant dealings. His life had long been messed up with events he had no control over, and he had resigned himself to his current life. That is, until a poster leads him to a familiar face he hadn't in years. Hitoshi Kiritani is an average teacher trying to live up to his principles and forget a traumatic past. But KAI has other plans: to make him fall into the same hell as him. "Fall further to the deepest depths."

(Source: Futekiya)