In Matthew Lewis’s *The Monk*, the demon who seduces Ambrosio is Matilda.
Matilda first appears under the name Rosario, disguised as a male novice so she can enter Ambrosio’s monastery and remain near him. After Ambrosio is bitten by a poisonous serpent, she nurses him and claims to save his life by drawing out the venom. Her apparent devotion becomes the first major step in breaking down his vows and self-control.
Matilda eventually reveals her female identity and becomes Ambrosio’s secret lover. When he turns his attention to Antonia, she offers supernatural assistance, including a magic myrtle branch supplied through a ritual invocation of Lucifer. This enables Ambrosio to pursue increasingly violent and immoral acts.
The novel’s final revelations make the answer especially clear: Lucifer tells Ambrosio that Matilda was a demon helping him. Readers sometimes confuse her with Antonia, the innocent woman Ambrosio later attacks, or Elvira, Antonia’s mother. Lorenzo is also important, but he is Antonia’s suitor, not a supernatural tempter.