Morgana

After the death of her father during her childhood, Morgana was taken in by Uther and raised as Arthur's sister (though there was a brief flirtation during season one). She was commonly known by the people of Camelot as the King's Ward and she used this status to get everything she wanted.

Morgana feels very strongly about magic, but not the way Uther does. She thinks persecuting a person who has magic is a harsh punishment, and when Uther sentences a particularly attaching young Druid boy, Mordred, to his death, she proclaims that she will for now on be his enemy.

It was not until Morgana was older that she learned that she possesses magic. Even when she began to experience seemingly prophetic nightmares, she tried to deny their magical quality, knowing that fate awaited her if it became known he had magic. It wasn't until she began displaying physical abilities as well (setting her room on fire and smashing a vase with her mind during season 2's "The Nightmare Begins") that she came to accept her gift. Although she may be powerful, her magic is less defined because she has little training with her powers and does not know of their true extent.

Just recently, (this secret was discovered in Episode 5, The Crystal Cave) while Morgana was semi-unconscious from falling down the stairs, she overhears Uther telling Gaius that she is Uther's Daughter and that Arthur is her half-brother. She is also half-sister of Morgause through their mother, Vivienne.

In episode 12 of season 3, Morgana and Morgause [with the help of Cenred's immortal army] take over Camelot, capture the citadel, imprison the king, and Morgana is newly crowned as Queen of Camelot. Her victory does not last long, however as she is in turn overthrown by Merlin, Arthur and his Knights. She subsequently flees from Camelot, taking the injured Morgause with her.