Who is Marco Campbell. from Avalon High and what is their importance?

Marco Campbell is Will's step-brother and the reincarnation of Mordred, the man who helped to destroy Arthur. Marco is a troubled young man who's father was killed in combat. A death some blame on Will's father, who married Marco's mother almost right after his father's death. Marco has been taken over by the darkness and sees it as his mission to destroy Will. Marco is cruel and cold, but seems to change after Will defeats the darkness.