I voted for Rosella because of the "Damsel in Distress" factor. See, here's the thing...
Cassima was kidnapped by Mordack. Even though she knew her way around the labyrinth and helped Graham escape from the dungeon and was friends with all of Mordack's monsters living in the maze.... she still stayed in the kitchen and was scared of everyone. She hardly seemed capable then.
Then, Abdul Alhazred captures her in her own tower. Yes, she sent out Sing-Sing so she wasn't isolated from the rest of the world. But she has guard dogs loyal to her following Alhazred's orders and she couldn't convince them she didn't want to stay in the tower? Really? REALLY? Then she actually tries to get Alexander to leave so she can, what, stay locked in a tower? It's not like anyone in that family has ever saved her before, right? Oh wait....
Anyways, she really doesn't do anything at all until Alexander hands her a dagger. She didn't have anything blunt and heavy in her bedroom to beat someone over the head with? I find that hard to believe. Anyways, all she did with the dagger was stab the bad guy in the shoulder -- if she was really all that capable, she'd have stabbed him in the neck after all of that. Seriously. Does. Not. Deserve. To. Live!
So, while she's brave and she's certainly adventurous enough to be a solid match for Alexander, I think our Damsel in Distress factor here proves she's incapable of saving herself. For me, that's an issue.
By my count, Rosella has been in a Damsel in Distress situation four times.
The first time, she volunteered to go to the dragon, hoping to save Daventry. The fact that Alexander had to rescue her is kind of beside the point given that the whole thing was her brave and heroic idea to begin with.
The second time, Lolotte locked her in the tower. Yes, Edgar gave her the key. Outside of that, Rosella rescued herself. Edgar didn't even give her any kind of immunity from the guards, she had to sneak around, get her stuff back, and slay Lolotte all by herself.
The third time, Mordack shrank her and the Castle Daventry and kept them in a jar. Yes, she needed Graham and Crispin to save her. But, seriously? It's not like Alexander, Valanice, or anyone else in the castle was much help. (They never address this but the chances it was just the three of them in that castle are, well, absolutely zilch.

)
The fourth time, she was turned into a troll and kept in the Vulcanix Underground. Not only did she find an ally all by herself, she got all the ingredients to become human again and then escaped Vulcanix Underground.
Damsel in Distress? I think not!
PS I really didn't find Rosella that spoiled in KQ7. I don't know why everyone thinks she is? Is it the whole thing in the beginning? If anything I thought Valanice was being ridiculous -- yes, she understandably wants Rosella married, but this is NOT the same Valanice who is so supportive of Alexander in KQ6. Heck, this is not the same Valanice who's all adventurous and totally worthy of Graham in the rest of KQ7! This is Valanice, the worrywort mother who doesn't understand that her children cannot be involved with boring people or they will lose their minds.
And, yes, Rosella does think a lot of things are gross in KQ7. But most of that stuff really IS gross. It's not like it stopped her from doing the things she needed to, she just commented that it was gross.