Annhild

Annhild was a childhood friend of Rein. It is said that while Rein might be the axe of Ragnar, Annhild was the hammer. While appearing cold-hearted to many, most of the Vikings that served around her considered her a valuable commander who was just cold-hearted to keep her units focused against the threats of Ragnarok. 
In reality; however, Annhild was a Jotunn double agent working alongside Rein to kill off all humans from within. During the battle of Loki's Decept, she was captured by Odin's forces. Where she is right now is currently unknown.