Kono Fukushuu ni Gal wa Iranai

Nozomi Kashihara's parents were killed when he was young, and he has lived a life as an assassin without knowing any human warmth. Betraying the "family" that raised him, he now spends his days as a serious high school student without any friends. But when he helped Shingu-san, a gal in his class, from some delinquents, they begin to form a relationship. But Kashihara can't help but suspect that she might be an assassin targeting him...?

(Source: Young Animal Comics, translated)