Melisandre

Melisandre, often called Melisandre of Asshai, is a character in Game of Thrones. She will be played by Carice van Houten and will debut in the second season.

Melisandre is a priestess of an eastern religion which is little-known in Westeros. She visited the island stronghold of Dragonstone and has become a close advisor of Stannis Baratheon and his family.

In the books
In the Song of Ice and Fire novels, Melisandre is a priestess of the red god, R'hllor, the Lord of Light, whose worship involves the idolization of fire and light. R'hllor is a popular deity in several of the Free Cities (particularly Lys, Volantis and Lorath) but is almost unheard of in Westeros.