The Green Knight
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 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.
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.