Avenger is the goddess of revenge, leader of the Demonic Beasts.

She is Leader of the Three Goddess Alliance, and a Servant who seeks the total eradication of humanity. Her class is Avenger. Avenger is a monstrous creature with hair composed of snakes and could be called the Queen of Demonic Beasts. She is based in the mountainous region north of Uruk.

(Source: Official site, edited)