Until the Annadil Crisis, the Pattern Knights were only rumors. But since then, several of them have come out of the shadows and into the light. One such is the Green Knight, who defends the Pattern of Arden, itself not known to exist until that time.
A chance violation of the laws of Arden during a maying trip placed Princess Hiko in the position of either compacting with the Green Knight (through sex) or fighting a duel with him, in order to deal with the breach of the law. To the surprise of many, she chose the sex.
This proved to be not a one-off thing, as in previous compacts the Green Knight had made with the Kings of Amber, but a lasting relationship, which eventually bore fruit in the form of Prince(ss) Morgan, their daughter.
The Green Knight can change his sex and appearance at will, but has largely persisted in his current form since compacting with Hiko. He generally appears as a deeply tanned man with long black hair and green eyes, clad in green armor and riding a brown horse.