Kuradeel

Kuradeel is a member of the Knights of the Blood guild assigned to protect Asuna, a duty he carries out with visible pride and more than a little overreach, frequently overstepping boundaries she never asked him to cross. When Kirito suggests he could do a better job of keeping her safe, Kuradeel takes it as a personal insult and challenges him to a duel.

His protective act conceals a much darker truth: Kuradeel is secretly a member of Laughing Coffin, the guild responsible for numerous player murders during the Aincrad death game. When his cover is finally blown, he reveals a genuinely sadistic streak, killing fellow Knights of the Blood member Godfrey without hesitation and turning on Asuna and Kirito once his double life is exposed.

His betrayal marks one of the more brutal turns of the early Aincrad arc, forcing Asuna to confront the fact that the guild meant to protect her had been harboring a killer from the start.