Parry, an orphan, is taken in and is accidentally adopted by a wizard who teaches him the benefits of white magic and how it can be used to help others. A musician and adept white magician, Parry plans on following in his father's footsteps when he is encouraged by his father, the sorcerer, to take a bride. Parry selects Jolie, seeing her potential despite her ragged appearance. Using his unique singing talents, Parry convinces Jolie that he means no harm. Taking her in, Parry and his father begin to teach Jolie the ways of wizardry and they begin to fall in love. With his father's blessing, Parry and Jolie wed and are about to start a life of bliss when they are attacked by crusaders of Christianity. Parry's father is killed in the attack and Parry escapes in bird form while his wife Jolie had gone ahead to warn her parents to go to the pre-determined hidden shelter. Unfortunately by the time Parry gets to town to check on his wife, she has been taken prisoner by the crusaders, who capture Parry himself shortly after he arrives. Working in conjunction with his wife, since he possesses a magical second sight, he frees them both but not before Jolie is slain by the dying Captain who was going to rape her. Taking off in horse form with Jolie strapped to his back, Parry arrives at the shelter and tries to heal her wounds but is lacking in medical supplies to save her. Parry watches as his wife dies in his arms. Vowing vengeance, Parry thinks the best way to escape from the villagers is to hide in plain sight, so he joins a monastery for sanctuary as well as a means to destroy the enemy. Soon after joining the Franciscan monks, Parry discovers that a new order, the Dominicans, are being formed with the express purpose of rooting out evil and heresy. Because of his keen mind and magical prowess (which he uses in secrecy), he becomes a feared inquisitor. During one of his many trips to stop Lucifer's campaign of Evil, Parry succumbs to the temptation of his ghostly wife Jolie inhabiting a physical body, thus violating his oath of celibacy.
As retribution, Lucifer sends forth Lilah (alternately known as Lilith), a demoness, to corrupt him. By using the toehold of his broken oath of celibacy and his own feelings of sexual desire and guilt, Lilah corrupts Parry to Evil. His intense desire for Lilah eventually leads him to corrupt the Inquisition itself. Upon his deathbed, Lucifer attacks Lilah; with his last vestige of strength, Parry manages a surprise attack against Lucifer, saving Lilah. Lucifer, taken off guard, succumbs. As a severely weakened Parry lays dying with only moments to live, Lilah tells him to claim the office before it finds Lucifer's successor, as well as to name the age he would like to assume (he chooses 25). Parry, not understanding what she's asking of him and wanting to honor this last wish before he succumbs to death, does as Lilah requests, and is suddenly transformed into the new Incarnation of Evil and takes the name Satan. (It is later explained that if no-one claims the office, it seeks out the most qualified person for that position. So if Parry hadn't claimed the office as Lilah had told him, it would have found the most evil person on earth to take Lucifer's place.) In For Love of Evil, several scenes from the previous books (as is the case with all the books in the series with respect to their incarnations) are shown from Parry's point of view. Parry also does not believe himself to be evil, but is simply fulfilling his function as an Incarnation.
It is rather ironic that Parry is not actually evil, he merely wishes to find a way to better facilitate separating good people from bad by using temptation. In fact, in the universe of "Incarnations of Immortality" Parry, as a personal favor to YHWH (the incarnation of the God of the Jews, called JHVH in And Eternity), takes steps to prevent the Holocaust from happening.
Upon taking office, Parry approached the other Incarnations in good faith, but was rebuffed and/or humiliated by them, since they assumed being the Incarnation of Evil he was naturally their adversary. Of them all, only Chronos offered friendship. This led to his enmity with many of their successors, though his friendship with Chronos and his various predecessors lasts for centuries until he intentionally goads and mocks the Incarnation as a means of getting him to annul the Holocaust.
He also attempted to meet with the Incarnation of Good to figure out how to best sort out which souls belonged in Heaven and which in Hell (Parry had no desire for souls that didn't belong in Hell to be there), but was not successful. The Incarnation of Good was too busy contemplating his own greatness to pay any attention (more on this in the novel And Eternity). Instead he strikes a bargain with the Archangel Gabriel: if Parry cannot corrupt one influential individual or her children or grandchildren in order to shift the balance of the world to evil, he must give up his quest. The individual is Niobe Kaftan—meaning he will have to wait six centuries before he can act.
Later he meets Orb, who he has decided to court so that he might defeat Good once and for all. However, he falls in love with her. When Orb becomes Gaea he asks her to marry him. At the wedding he surprises everyone by singing Amazing Grace, which causes him to vacate his office. Since no one takes it, the office elects the most evil person on earth, a cruel murderer and child rapist. After a match of wits, Parry eventually reclaims his office, to the relief of the other Incarnations who would prefer a fair evil than a ruthless one. They realize that Parry is not really evil—he just hates the system of soul placement.