Arcueid Brunestud

Arcueid is a vampire hunting others of her kind. She "convinces" Shiki to help her deal with the vampire in the town as she used up most of her power because of him.

Arcueid is shown to be a whimsical individual who, although knowing the basics of the modern era, does not seem to know about social norms. Her behaviour is almost child-like, due to the lonely nature of her up bringing, although she is over 800 years old, she spent most of it asleep and knows nearly nothing about the world or how to interact with others.

Second paragraph edited from Typemoon Wikia