My Little Monster(2008)

Mizutani is a self-absorbed overachiever, concerned only with maintaining the highest grades in school. Haru is an impulsive short tempered brute, who scares everyone with his explosive bursts of violence. Haru gets suspended on the first day of school when he encounters some bullies harassing a student and dispatches the bullies with great bloody violence. Mizutani is tasked with delivering school materials to Haru who interprets this as an act of friendship and latches on to Mizutani, much to her dismay. And so begins a strange and potentially combustible relationship!

(Source: Kodansha USA)

Note: Includes 17 extra chapters.