Sasha

Sasha was one of Brimir's familiars. She made a pact with Brimir to allow him to experiment on her but he made her his familiar, something Sasha was not pleased about. In his defense, Brimir meant no harm to come to Sasha when he made her his familiar.

Sasha killed Brimir by stabbing Derflinger into his heart while he tried to kill off the elves for reasons not yet revealed as of the 19th light novel possibly she was an elf.

Trivia

 * Despite already being a Gandálfr, Sasha was Lífþrasir as well, similar to Hiraga Saito 6000 years later.
 * She was the person who made Derflinger, as well as the first person to use it.