Waldemar Fitzurse is Prince John's primary counselor during his rebellion against his brother, King Richard. Fitzurse is a fairly standard character type: the older, wily advisor. He's like Gríma Wormtongue in Lord of the Rings (but less slimy): silver-tongued but totally disloyal and amoral.
Fitzurse is always working in the background of this novel, greasing palms and persuading people to join Prince John's side. When John first starts to lose his nerve when he hears rumors that his brother is back in England, Fitzurse encourages him to stick to his rebellion and not freak out. Fitzurse calls himself John's advisor, but he's really the brains behind this whole revolt. Without his manipulations, John would probably have lost his nerve way back around the time of the tournament at Ashby-de-la-Zouche.
But in his own mind, Fitzurse doesn't think much of Prince John, De Bracy, or any of the others he has drawn into his web of plots. He mutters to himself at one point, when De Bracy is hatching this stupid plot to kidnap Rowena and marry her:
"Like a true knight?" repeated Fitzurse, looking after [De Bracy]; "like a fool, I should say, or like a child, who will leave the most serious and needful occupation, to chase the down of the thistle that drives past him. – But it is with such tools that I must work; – and for whose advantage? – For that of a Prince as unwise as he is profligate, and as likely to be an ungrateful master as he has already proved a rebellious son and an unnatural brother. – But he – he, too, is but one of the tools with which I labour; and, proud as he is, should he presume to separate his interest from mine, this is a secret which he shall soon learn." (15.25)
Fitzurse thinks De Bracy is a moron and Prince John is an unprincipled jerk – but he'll use both of them, and anyone else he needs to, to achieve more power for himself. What Fitzurse wants most of all is authority behind the scenes.
Even if Fitzurse is the driving force behind this rebellion, he finally loses his nerve in the end. When De Bracy escapes the outlaws after Torquilstone and confirms that King Richard is back in England, Fitzurse thinks about fleeing the country. He talks about this plan openly in front of Prince John, though he admits it would be difficult: he is old, he has a daughter to think of, and there are troops loyal to King Richard I standing in the way.
Prince John is horrified by Fitzurse's complete lack of loyalty – though he really should have suspected it, since Fitzurse had to be disloyal in the first place to desert his king and join Prince John's side. John reminds (fictional) Fitzurse that he's the son of (real-life) Reginald Fitzurse, one of the four knights who killed Saint Thomas à Becket on the assumption that that's what King Henry II, Prince John's father, wanted them to do.
Reginald Fitzurse was loyal to his king, almost too loyal. His fictional son Waldemar protests that he's just as honorable as his father, and he stands by Prince John (reluctantly). But clearly, with this outright show of disloyalty among his counselors, even Prince John must know that his rebellion is breaking apart before it has even really started.
Besides being power-hungry and disloyal, Waldemar Fitzurse is also really, really proud. We first see this when Prince John reminds Fitzurse of his father's extreme loyalty, which pisses Fitzurse off. His pride really comes out at the end of the book, though, when he leads a small band of assassins to try to pick off King Richard I while he's still traveling in the forest in disguise.
The assassination attempt fails because King Richard can call on his outlaw buddies to help him fight the assassins off. Richard subdues the head assassin ("the Blue Knight") himself, but when Wamba pulls off his helmet, Richard is utterly stunned to see that it's Waldemar Fitzurse. Fitzurse tells him straight out that the reason he's so dead set against Richard is that he refused to marry Fitzurse's daughter, Alicia. Is his daughter not good enough for Richard? Isn't Fitzurse's bloodline as noble and Norman as anybody's? So it turns out that, for all of his power-grubbing and manipulation, Fitzurse's main reason for hating King Richard is personal.
Richard is surprisingly understanding of the reasons for Fitzurse's resentment, so he doesn't have him executed. Instead, Fitzurse just gets banished – which seems like a light punishment for attempting to kill a head of state. But King Richard I is a forgiving guy.