The Green Knight
The Green Knight of Bretonnia is the most celebrated of figures in Bretonnian folklore; a fey knight with ivy-wreathed armor and shield, glowing eyes and a green-bladed sword, the Green Knight is a protector of Bretonnia who invariably appears when the forces of evil are despoiling the land. When not battling beastmen or Orcs & Goblins, he often acts as a final challenger to Questing Knights; forcing them to duel him and prove their worth before they are allowed to drink from the sacred waters of the Grail and be transformed into Grail Knights.
Holy warrior of Bretonnia, may or may not be King Arthur, is a ghost or a daemon (a nice one mind you). Every time he gets killed he always come back later on. Seriously in one fluff story he had his head lopped, bent down, picked up, bade the hero a how do you do and left.
Has a day job as a professional ass rapist. Seriously, the dude destroys all in his path when he hits the table. In many ways he's a non-Chaos counterpart to Doomrider in that he shows up, wrecks enemy shit, then leaves.
Totally NOT an Expy from Sir Gawain and the Green Knight. Is not Sean Connery either. If you get this reference, congratulations, you're hardcore.
Who he is exactly perhaps only the fey enchantress knows but she ain't telling anyone, the teasing harlot.
GW has finally lifted the lid on him, in the Nagash supplement for The End Times. He's Gilles le Breton; Bretonnia's equivalent to Sigmar and totally-not-King Arthur-we-swear-guys, resurrected by the Lady of the Lake. He becomes Bretonnia's new king after Louen's death and sets out to purge Bretonnia of the taint of Chaos.
He is based on the Green Knight, a magical warrior who appears in the Arthurian mythos. Which you'd only expect, since Bretonnia is literally the grimdark take on King Arthur and his Knights of the Round Table.
Interesting to note is that the tale of the Green Knight is based on old testament Satan (Satan translating as something equivalent to rival or sparring partner.) Put that in your role play and eat it.