Miriam, a painter, is of mysterious origins and nationality. She is variously conjectured to be a German princess, the heiress of a Jewish banker, the wife of an English nobleman, or the daughter of a southern planter—English but with “one burning drop of African blood in her veins” (19). Her looks are dark and beautiful. She has a vivacious, strong, outgoing personality and elicits comparisons to such biblical and historical characters as Rachel, Judith, Cleopatra, and Beatrice Cenci. Hawthorne often describes her as smiling in a mischievous way, and she is secretive about her past until finally opening up to Kenyon in Chapter 42.

Although fundamentally of an innocent and moral nature, Miriam has a mysterious attraction to evil and becomes entangled with it when Brother Antonio begins to follow her around Rome. The novel also hints that she was earlier involved with another crime, of which she was innocent but which forced her to run away from home.

When Donatello confronts Antonio, she seems to encourage him through her facial expression to kill him; later, she acknowledges that in her heart she wanted Antonio dead. Along with Donatello, she is forced to go into hiding from the law. Still later, Miriam tells Kenyon that she wants her love to redeem Donatello and make him into a better and deeper person.