Guinevere
As is often the case, Guinevere is portrayed as a woman who is torn between following her heart and obeying her will. Although she loves Lancelot, she has sworn herself to Arthur. Her mistake is in believing that her will is stronger than her heart. Throughout the movie, she engages in a hopeless struggle to control her heart.
MP3 Guinevere: If there is any honor in you, promise me never to do that again. Lancelot: I don't know about honor, but I promise you, I won't kiss you again 'till you ask me to. Guinevere: And that I never will.
An interesting condition of the story is Guinevere's freedom to marry whomever she chooses. Portrayed as an strong, independent woman, she has no father forcing her into marriage, nor does she seem to have any motivation for marrying anyone other than a husband of her own choosing. Arthur even promisses to protect Guinevere's kingdom whether she marries him or not. This unprecidented freedom makes her betrayal all the more damning, because it was perpetuated from a position which she herself chose.