Morgan le Fay
Like Arthur, the character Morgan le Fay gets relatively little face time in the poem, but gets a long description when she’s first introduced. Unlike Arthur, however, Morgan turns out to be the mover and shaker behind the plot, the person who sets its events in motion on a whim.
When we first meet her, it’s in the company of the beautiful Lady Bertilak, against whom the narrator upholds Morgan as a portrait of ugliness: she’s night to Lady Bertilak’s day. Whereas that beautiful lady bares all, the elderly Morgan covers herself in veils from her head to her toes, so all that’s visible are her eyes, nose, and lips, which were "repulsive to see and shockingly bleared" (964).
This being a medieval romance and all, the fact that Morgan’s physical appearance is ugly means something about her insides, too. And sure enough, at the end of the poem the Green Knight tells Gawain that it was Morgan that enchanted him and sent him to Arthur’s court, in order to test Arthur’s knights and frighten Guinevere to death. The Green Knight’s not necessarily judgmental of Morgan: he simply tells the truth about her behavior and, furthermore, calls her an "accomplished scholar," although she’s a student of the occult arts (like sorcery) rather than of more morally legitimate disciplines.
We learn that Morgan once had "love-dealings’" (sex) with Merlin, which is how she acquired her learning. Like Lady Bertilak, then, Morgan is something of a seductress. Yet it’s really Morgan who wields the power over both Lady Bertilak and Lord Bertilak. It’s up to us to decide whether that’s a bad thing or not. Gawain probably thinks it's bad, given what she’s just subjected him to. He would probably class Morgan amongst the wicked women he describes in his anti-feminist diatribe. Yet like so many of the characters in Sir Gawain, Morgan is more complex than she might appear. After all, hasn’t Gawain just learned an important lesson, albeit a painful one, from his test? Could there be a method in Morgan’s madness after all?