The tricky part about any Bretonnian is that it’s hard to get one character who could conceivably have three career options. It’d be easiest with a peasant, but c’mon - if you’re gonna have a Bretonnian, we all know their whole fantasy revolves around being knights.
For a Bretonnian Knight, you’d have to give them a proper knightly class (errant, grail, whatever), who has no ranged options. You could also do a Son of Bretonnia, who are the male children with magical power, raised by the Lady of the Lake to be her personal knights. They’re very magical, so this class could have some kind of blessings or prayers as ranged weapons. And finally - a Herrimault. Basically a Robin Hood-esque figure.
Backstory would have to be that he was born with some magical talent - we don’t know that every single last magical baby born is discovered (this idea rides on this), thus in one scenario he became a normal knight without ever realizing he had magical aptitude. In another, he is taken by the Lady of the Lake. And in the last he was wrongfully accused of a crime or whatever, and becomes a Herrimault to fight against injustice.
Also, at this point, I agree that a Grail Knight would not be out of line for the existing heroes. Grail Knights aren’t even heroic units, and several of the characters’ classes are. Plus, they must have wills far beyond the average to not be riddled with Nurgle diseases and chaos corruption by now.
@Tamren - that’s a good thought, about a new character not necessarily having three careers, though I think that if they’re already making a new character, they might as well squeeze as much out of it. Many assets (weapon models, voice lines) could be recycled.