Leila

Conspicuous as the sole woman of the group [The Marcus Brothers], although she isn't related by blood to the Marcus Brothers, they consider her their little sister. She fights using a very powerful gun. Leila joined the Marcus Brothers to avenge her parents, as her father was killed trying to rescue her mother from vampires, who in turn, turned her into a vampire too. The town's people stoned Leila's mother to death while a young Leila watched, helpless.

http://myanimelist.net/character/1020/Leila