Prior Aymer is the leader of the monks at the Priory of Jorvaulx. (A "priory," like a monastery or a convent, is a place where people under religious vows go to live.) As a guy who is pretty high up in the local church leadership, you might assume Prior Aymer is a serious man of faith. Not so much – no matter how religious his job title sounds, Prior Aymer is all about drinking, sex, and money.
Prior Aymer is also quite open about his worldly tastes. He dresses to the letter of church law but not the spirit: all his clothes and ornaments are made of expensive materials. He frequently visits local ladies for decidedly non-religious reasons. He tells Cedric outright that he only drinks healthy stuff like milk when he's at home in the Priory; otherwise, he's all about the wine. In a way, we have to hand it to Prior Aymer: he may be on the side of Prince John and the other rotten Normans, but he is no hypocrite like Albert Malvoisin. He's surprisingly honest about the fact that being a prior is just a job for him, rather than a way of life. Despite his job title, he enjoys fun just as much as the next guy.
Prior Aymer and Bois-Guilbert are friends, but Prior Aymer is a much better politician. When both of them wind up at Cedric's house for the night, Prior Aymer tries to soothe Cedric's feelings whenever Bois-Guilbert says something anti-Saxon. Unlike Bois-Guilbert or his other Norman friends, Prior Aymer is neither especially proud nor particularly violent. He just likes enjoying himself as much as possible, which is easier to do when people aren't fighting.
Prior Aymer eventually gets captured by the outlaws of Sherwood and agrees to pay them a heavy ransom. The main purpose of this adventure in the forest is to give Scott a reason to bring Prior Aymer and Isaac together at just the right time. After all, the outlaws also rescue Isaac from Torquilstone, but they won't let him go without a ransom. Prior Aymer is on hand, also captive, when Isaac hears that Bois-Guilbert has kidnapped Rebecca. For a fee, Prior Aymer agrees to write a letter to Bois-Guilbert asking him to let Rebecca go. It's this letter that Lucas Beaumanoir finds when Isaac comes to see him, which spills the beans about Bois-Guilbert's secret love for Rebecca.
Wow. Scott is really straining the realism of the novel with this plot line. Think of the odds of Prior Aymer and Isaac both being in the same part of the forest, captives of the same outlaws, when someone tells Isaac the fate of Rebecca. Then what are the odds that Isaac bumps into Beaumanoir with the incriminating letter in his hand? Scott is clearly trying to set up Rebecca's final witchcraft trial, but it strikes us as a little hard to believe that he's using Prior Aymer to do it. Still, no one ever said Ivanhoe is supposed to be a realistic novel. It's a romance with knights and Robin Hood – we suppose we can forgive it for being a bit farfetched.