Raven's half-sister and Edgar's dear friend. Edgar would do anything for her if it will make her happy. She is a beautiful woman who has fine fighting skills (though she is not on par with Raven), with short black hair, snow-white skin, and a womanly alllure that "is not dimmed by her wearing male clothes" most of the time.
She was in love with Edgar, and the Prince used this fact to his advantage and turned her into a spy to be used against Edgar. She is the one who tells Lydia about Edgar decieving her once again and how Edgar is probally determined to sacrifice Lydia's life to become the Earl. Later she confesses her betrayal to Edgar and commits suicide by jumping into the sea.Later, she is revived by the Selkies on Ulysse's command, and becomes a Selkie. After reuniting with Edgar's group, she serves him for a further amount of time before betraying him yet again. The reason for her betrayal is yet unknown, but she claims to Kelpie that her soul's master is and always will be Edgar. In addition, she attempts to discreetly tip the scales in Edgar and co.'s favor when she can, such as by dropping hints to Kelpie; she tells him that there's a reason why she must not be kicked out of Prince's organization yet, but does not state it openly.
While she does not openly express her affection for Raven, she has been shown to worry about him.
Later on, Ermine relates to Lydia that when they were still under the Prince's organization, she was the woman "given" to Edgar (as his sexual partner). Edgar, however, refused to touch her becuase he understood that it was the Prince's way of training Edgar's sexual inclinations to match his own. When the Prince realized this, he raped her in front of Edgar.
Lydia believes that Ermine is the one whom Edgar truly loves (She bases this conclusion on a five things, like how Edgar is never flirtatious towards her - Lydia takes this as a sign that Edga lovers her so deeply he cannot bring himself to be shallow with Ermine - as well as him muttering her name before falling asleep while holding Lydia). As such, even after she is vaguely conscious of her feelings towards Edgar, she continues to reject acknowledging them as it would be too painful for her to be with Edgar if his heart is with another.
-Lydia once saw Ermine and Edgar kissing in Edgar's room
-She is Raven's half sister
-When she seemingly dies she asks Raven to let go of her hand when falling of a cliff