Agravain

Agravain, one of the Knights of the Round Table, is the son of Morgan, making him the brother of Gawain, Gareth and Gaheris, and half-brother of Mordred. He was also an assassin employed by Morgan le Fay to serve as one of the knights. Acting as the secretary of the Round Table, he was one of the few to find out the secret of the King's gender. The affair between Lancelot and Guinevere was discovered by him, and he used the secret of the king to threaten the queen. This act was, however, also his undoing, as Agravain was killed by Lancelot, who was overtaken by rage.

(Source: TYPE-MOON Wiki)