There are lots of novels out there that pick on one group as evil or wrong – they're 'Other,' or different from 'us.' And there are plenty of characters in Ivanhoe who try to treat Isaac and Rebecca as Other, less than human, because they are Jewish. Not only does Scott portray this anti-Jewish prejudice, but he also tries to imagine how being treated as Other would affect the psychologies of his Jewish characters.
For example, Isaac is so used to being abused by Christians that he always assumes they are out to get him. (Sadly for Isaac, he's often right.) While Rebecca worries less than her father, she also exhibits humility toward Christian ladies like Rowena. Life has taught her to expect prejudice from the people around her, and she bears it patiently when it comes her way. Isaac and Rebecca's humility is at least partly a symptom of how badly they have been scarred by anti-Jewish hatred.
In a novel that strongly emphasizes the moral importance of loyalty, Brian de Bois-Guilbert's lack of loyalty to anyone marks him as Ivanhoe's ultimate Other.
By idealizing Rebecca, Ivanhoe exposes the offensiveness of anti-Jewish prejudice. At the same time, her extremely positive characterization draws attention to the otherwise stereotypical portrayal of Isaac and the other Jewish characters in the book.