Like a mother who flagrantly dotes on her wildest child, Joanne Harris has always lavished love on her darkest characters - think of Renaud in Chocolat, Brismand in Coastliners. Now, in Holy Fools, she has given birth to the greatest villain of them all: Guy LeMerle, a cut-throat so disloyal, so morally empty, that he bears only one comparison - Keyser Soze in The Usual Suspects. Yet I found myself idly musing, "Well, I wouldn't kick him out of bed for eating biscuits..." Thus does Harris entice us to confirm the central creed of her latest novel - that evil can entrance the human soul.
Holy Fools is the story of Juliette, a street acrobat who becomes infatuated with Guy LeMerle, a mysterious, corrupt and beautiful young man who leads his outrageous, satirical dance troupe from town to town in plague-torn, politically volatile 17th-century France. When her treacherous lover abandons her to certain death, Juliette escapes with her little daughter to Sainte Marie de la Mer, a remote convent run as an undemanding - almost pagan - commune. Following the murder of Henri IV in Paris, a new abbess arrives determined to infuse the nuns with strict religious zeal. Re-enter LeMerle, masquerading as a zealous and wildly handsome priest, who sets out to win the hearts and loins of every adoring woman in the place.
For personal reasons of his own - pride, rage, self-amusement - LeMerle, watched by a mesmerised Juliette, persuades the innocent nuns that they are possessed by the devil, and then sits back to admire their ensuing descent into mass hysteria and the fires of hell. Egged on by this preposterous priest, the innocent nuns gyrate and twitch and expose themselves, vying to be possessed by demons, revelling in the imagined depths of sin and oblivious to the fact that there is only one devil here - the man LeMerle, smiling and nodding as he warms his hands at the fires of human frailty. Juliette, armed only with truth and rationality, will take him on, and perhaps defeat him in the short term, but she cannot uproot the seed that he has planted in her womb and soul.
Joanne Harris fought hard for LeMerle. Four months before the book came out she said that "LeMerle has already caused more controversy than any other character I have ever created," adding that "I don't see him entirely as a villain. In fact, he is a kind of existential hero." Championed by his stubborn author, LeMerle survived and strutted forth on the page unchanged and unrepentant.
Of course, Harris was completely right to stick to her vision. LeMerle is evil triumphant, but he triumphs because stupid, uncalculating innocents can hear a man intone the scriptures and believe it is the voice of God; because we are gratified by our helplessness in the face of sin; because we set out to be possessed by lies; because we think truth is boring and the devil fun.
Harris always pays careful attention to physical and social detail, and writing a "historical" novel has brought out the best in her descriptive style. The subject matter even sits well with the odd authorial overspill into purple melodrama. No surprise, then, if this novel makes it to the screen. Also no surprise if the more ruthless god of film makes Hollywood mincemeat of LeMerle, bringing him repentant to his knees.
· Helen Falconer's novel Sky High is published this month.