Mahou Tsukai Kurohime

A master gunman named Zero searches the land for the legendary Kurohime. She saved him as a child, impressing upon him the importance of justice and helping others. While on his journey he meets a young girl named Himeko and is suddenly attacked by overwhelming forces. Outnumbered, outgunned, and out of luck, he makes his last stand in order to protect this "helpless" girl. Little does he know, this girl has an incredible secret.

(Source: Viz Media)