Zadkiel

Once a powerful member of the heaven's council she has since left and been labeled as a traitor. 

She once took on the task of teaching Nuriel about her duties as a Seraph while her parents were too busy with their own duties. 

Nobody is sure where she vanished to, since there's been no reports of her working with the daemons but the heavens have done their best to erase the traitor's name from all scores.