Who was King Arthur's treacherous sister?

Morgan le Fay, alternatively known as Morgain, Morg[a]ne, Morgant[e], Morge[i]n, and Morgue[in] among other names and spellings, was known as King Arthur's sister and as a powerful enchantress in the Arthurian legend. In several versions of the legend, Morgan is a character who is just a half-fairy and half-sister of mythic King Arthur.

In some of the stories about le Fay, her personal heritage has granted her immortality, and she uses a good slice of her time to master the mystic arts. She occasionally tries to control each and everyone in the known and unknown world/universe with her super abilities.

Morgan's actual character may have been rooted in Welsh mythology as well as other earlier myths and historical figures. The earliest account, by Geoffrey of Monmouth in "Vita Merlini", refers to Morgan in conjunction with the Isle of Apples (Avalon). This is the place where Arthur was carried after being fatally wounded in the Battle of Camlann. There, and in the early chivalric romances by Chrétien de Troyes and others, her chief role is that of a great healer. It is Chrétien who establishes her as Arthur's supernatural elder sister.