Jacques Fitzdotterel is a man Front-de-Boeuf mentions in passing to Isaac when he's threatening him at Torquilstone. Apparently Isaac took this man to court for calling Isaac a "usurious bloodsucker" (23.36). (This would be someone who loans money with insanely high interest to make an unfairly huge profit.) Front-de-Boeuf uses this case as proof that Isaac doesn't deal fairly in his business negotiations.
Kirjath Jairam is a rich Jewish man living in the city of Leicester. Isaac arranges to borrow a fine horse and good armor from Kirjath as a reward for Ivanhoe's help escorting Isaac safely to Sheffield from Rotherwood. Ivanhoe wins back the price of his horse and armor during the tournament, so Kirjath gets paid back for his loan and everybody's happy.
Isaac refers several times in the novel to his beloved wife Rachel, who died before the events of the book took place. Isaac also says that Rebecca is "the image of my deceased Rachel" (22.41).
Reuben appears to be a servant in Isaac's household. After Rebecca passes Gurth her money to pay for Ivanhoe's armor, she orders Reuben to escort Gurth out and lock the door behind him. Reuben also helps Rebecca tend to Ivanhoe when he is recovering from his wounds at her house.
Scott gives Isaac a friend who counsels him to be careful of Lucas Beaumanoir and who comforts him when Rebecca is abducted by Bois-Guilbert. The thing is, Scott can't quite decide what this friend is called: should it be Rabbi ben Samuel or Rabbi ben Israel? Either way, he's a nice guy, and he represents the larger Jewish community in York, of which Isaac is a part.
A second servant of Isaac's.
Zareth is one of Isaac's family members in the nearby town of Sheffield. He helps arrange the suit of armor and horse that Isaac loans Ivanhoe for his tournament at Ashby.