Isn't the princess fighting against her own role as a saint as much as she can? Well, as much as she can within reason, without abandoning her position. She's Princess Fanservice, which isn't exactly the kind of appearance one would expect from a princess-saint. Maybe she's actually a really lewd person, but is frustrated because she can't go that far as a saint. That makes her angry at Leold, who's free to change himself, if he so chooses.