Spoiler Alert! The Green Knight is the wild side of the eminently civilized Lord Bertilak. Yup, that’s right, they’re the same character, although we don’t find this out for sure until the end of the story. It's kind of like Dr. Jekyll and Mr. Hyde, except that neither personality is evil.
For those of us who have been paying attention, Lord Bertilak's dual personality may not come as much of a surprise. After all, the physical appearance of Lord Bertilak, with his long beard, large stature, and firm stance is suspiciously similar to that of a certain other large, bearded guy we meet at the beginning of the poem. Not only that, both men convince Gawain to play a game of exchanges with them, the Green Knight’s being an exchange of axe-blows, while Lord Bertilak’s involves an exchange of winnings. OK, so we’ve established that it makes sense that these two are one and the same character given their similarities. But what about their differences, which also happen to be pretty revealing?
Well, as we mention in other parts of our discussion ("Symbolism, Imagery, Allegory" and "Quotes: Man and the Natural World"), the Green Knight’s, er, greenness represents his connection to the natural world. The holly branch and axe that he carries and the Green Chapel’s wild, natural location adds to this connection. There’s no doubt that this guy’s close to nature. He helps Gawain get in touch with his animal nature by showing the very civilized knight of the Round Table that he has the same sort of survival instinct that animals do. The Green Knight proves to Gawain that basic survival is a more powerful drive than he thought.
But what about the Green Knight’s alter-ego, Sir Bertilak? His elegant palace is full of luxury: beautiful tapestries, silken bed linens and draperies, fine furs and embroidered garments, gourmet cooking... all of the finer things in life. These are all the trappings of civilization. Sir Bertilak’s warm hospitality toward Gawain marks him as a practitioner of courtoisie, or courtly behavior, a defining characteristic of this culture. And although Sir Bertilak gets back to nature by hunting some deer, a boar, and a fox, the descriptions of those hunts represent them as systems that impose a human order on the natural world – man’s triumph over nature, rather than his connection with it.
So, what happens at the end of the poem, when we learn that Sir Bertilak and the Green Knight are one and the same character? True, Sir Bertilak’s wild side is not necessarily his choice: the sorceress Morgan le Fay has enchanted him in order to frighten Guinevere and test Arthur’s knights. But on the other hand, why can’t one person incorporate both a civilized and a wild side? After all, as Gawain learns through the beheading / exchange of winnings games, even the most civilized knight has a bit of the animal in him. Sir Bertilak / The Green Knight may exist to teach Gawain (and us) this lesson not only through the games he plays, but also in the form of his two diverging characters.