In the tale of Sir Gawain and the Green Knight, there are many things about the Green Knight that are noteworthy. Perhaps the most impressive is that after Gawain cuts off the knight's head, the green giant picks it up, reminds Gawain of his promise and departs—much to the amazement of the people assembled in King Arthur's court at Camelot.
The Green Knight is indeed something to behold—very unusual in his appearance. He is large and he is completely green—not only his clothing but his hair as well. The narrator notes that he is...
...an awesome being, in height one of the tallest men in the world.
His neck and body are square and thick, and his arms and legs are very long. The narrator notes that he is...
...withal the handsomest, in spite of his bulk, that ever rode...
The knight is also slender in build, and...
He held himself in hostile fashion and was entirely green.
His hair is not short—it is described as "locks," which indicate some length—actually reaching past his shoulders. His beard is compared to foliage...
...as big as a bush [that] hung over his breast.
He is richly attired in beautiful clothing, adorned with white fur, and covered in material that, like the man, is green. He and his clothing are clean. His raiment is fantastic, covered with jewels, as is the horse, which is also covered in silk. Everything on man and beast is skillfully embroidered. (The man's horse is beautifully "dressed," and is as green as the knight himself.)
The Green Knight wears no armor, but carries a "holly twig" in one hand and an axe in the other. His color, massive size and challenge to the knights at court that New Year's Eve, make for just the kind of adventure Arthur was said to wait for, before beginning the meal.
In Sir Gawain and the Green Knight, the Green Knight first enters into King Arthur's castle during the Christmas/New Year's celebration. Curiously enough, the king will not eat until all of his guests have been served or without witnessing a challenge (like jousting) or hearing an "uncouth tale of some adventurous thing." It just so happens that all of King Arthur's guests had been served their food when the Green Knight enters his castle.
The Green Knight was referred to as such based upon the fact that the man was green:
At the hue of his noble face
he carried himself in hostile fashion
and was entirely green.
Outside of his color, the Green Knight was considerably large.
than there burst in at the hall door an awesome being,
in height one of the tallest men in the world;
from the neck to the waist so square and so thick was he,
and his loins and his limbs so long and so great,
that half giant I believed him to have been,
or, at any rate, the largest of men.
The Green Knight was clothed in green clothing, bedecked with fur and jewels. Not only was the knight green, his horse was green as well.
Outside of that, the Green Knight carried a holly twig in one hand and an axe in the other. The Green Knight was so impressive, and no person in Arthur's kingdom had ever seen such a being, that they all believed him to be a "phantom."
All were amazed who stood there, and stalked nearer to him,
with all the wonder in the world what he would do;
for many marvels had they seen, but such never before.
Therefore for phantom and faery the folk there deemed it.
Therefore, the Green Kinght's appearance was unusual given his great stature, his green color and his green horse.