Obscure trivia: Who played a girlfriend of David Boreanaz and a vampire killer?

Sarah Michelle Gellar is a good answer, but also Katheryn Winnick (with two characters).  

Winnick played a girlfriend of Boreanaz in Bones, for a short time. 

And she was a vampire hunting other vampires in one episode of The Gates.

The beautiful Canadian actress Katheryn Winnick often plays secondary roles in movies and series.  I wonder if she will get a more important role someday.