Dokkabei

She is Dokkabei(yeah from the Korean legends, creatures who can get really scary when they wear mask), so apparently you saved her when another monster race was going to use her as a sacrifice to summon a demon, since then she has been following you(she even gave you her dokkabei mask-it's somewhat like this 👹, which can only be given once to the person they love), what she wants is revenge, revenge from those who burned her village making her the only surviving dokkabei