Melisandre, often referred to as the Red Woman, is a Red Priestess in the religion of R'hllor, the Lord of Light, and a close counsellor to Stannis Baratheon in his campaign to take the Iron Throne. After Stannis Baratheon's death at the Battle of Winterfell, she revives Jon Snow after he is murdered by various members of the Night's Watch, believing him to be The Prince That Was Promised, and serves Jon as an adviser until she is banished from the Northafter the Battle of the Bastards when Jon learns that she sacrificed Shireen Baratheon and countless innocent people while in Stannis's service. However, she later unites the newly crowned King in the North with Daenerys Targaryen, believing they both have an important part to play in the Great War.
(source GOT wiki)