@Anitvenomz
from what i have understood :
she is fragment of Anastasia's soul.
She was one of the former comrade of Claude (
the werewolf) when he still was the hero of the earth spirit empress (
cf chapter 15 page 18 )and a childhood friend who was like a big sister for him (
cf chapter 9 pages 3 and 4).
But one year before the start of the story (
cf chapter 15 page 18 ) probably after the party killed one of the 8 demon lord of this world (
cf chapter 16 page 8), she seemed to had become mad, killed some people (i don't know if they were the other party members or not) and torn off Claude limbs (
cf chapter 9 page 4 to 7) and with other party member(s) cursed him (
cf chapter 15 page 18 ).
Claude believed to have killed her already (
cf chapter 15 page 19) (he was already missing one leg and one arm at that time and still managed to do it...).
But since her power was to put fragment of her soul into living beings (generally monster) to control them (
cf chapter 15 page 18 ), even though Claude killed the main body, she is still around, and still want to kill him.