It's established in the film that Mordo's morals are rigid; he feels betrayed when he learns that the Ancient One tapped into Dark Dimension magic in order to extend her life, even though extending her life allowed her to fight evil for centuries. (cbr.com)
In the third act of Doctor Strange, Mordo's rigid sense of morality and his idealistic faith in magic are deeply shaken by the revelation that his mentor, the Ancient One, has actually been drawing her power from the forbidden Dark Dimension. (slashfilm.com)
Really, all of the other roles on this list are a long shot compared to Clea, Doctor Strange's partner and occasional love interest who is also the Sorceress Supreme of her native Dark Dimension. (cbr.com)