Emily St. Aubert: Much of the action takes place from her point of view. Emily has a deep appreciation for the sublimity of nature, which she shares with her father. She is unusually beautiful and gentle with a slight, graceful figure, fond of books, nature, poetry, and music. She is described as extremely virtuous, obedient, resourceful, brave, sensitive, and self-reliant. Her childhood home is La Vallée. Her sensitivity leads her to dwell (often in tears) on past misfortunes and to imagine, with dread, troubles that might befall her in the future. She is given to writing verse, selections of which punctuate the novel.
Monsieur St. Aubert: Emily's father, who dies early in the novel while he, Emily, and Valancourt are travelling. He warns Emily on his death bed to not become a victim of her feelings but to acquire command over her emotions. His unaccountable relationship with the Marchioness de Villeroi is one of the novel's central mysteries.
Valancourt: The younger brother of the Count Duvarney, Valancourt forms an attachment to Emily while travelling with her and her father through the Pyrenees. He is a dashing, enthusiastic young man with a noble character, on furlough from the army when he meets Emily. St. Aubert considers Valancourt a desirable match for Emily, though Valancourt lacks wealth.
Madame Cheron (later Madame Montoni): St. Aubert's sister and Emily's aunt. Madame Cheron is a selfish, worldly, vain, wealthy widow living on her estate near Toulouse when Emily becomes her ward after St. Aubert's death. She is contemptuous and cold, even cruel, to Emily at first, and thinks solely of herself: but near her death, when Emily patiently and selflessly aids and comforts her, she softens slightly towards her.
Montoni: The prototypical Gothic villain. Brooding, haughty, and scheming, he masquerades as an Italian nobleman to gain Madame Cheron's hand in marriage, then imprisons Emily and Madame Cheron in Udolpho in an attempt to acquire control over Madame Cheron's wealth and estates. He is cold and often cruel to Emily, who believes him to be a captain of banditti.
Count Morano: Introduced to Emily by Montoni, who commands that she marry Morano. Emily refuses but Morano continues to pursue her in Venice and later Udolpho. When Montoni finds out that Count Morano is not as rich as he hoped, he abruptly withdraws his support from Count Morano's suit. Morano attempts to abduct Emily twice, but both attempts fail.
Annette: A maid who accompanied Madame Cheron from France. Annette is inclined to exaggeration and superstition, and is talkative, but she is faithful, affectionate and honest. She is in love with Ludovico.
Ludovico: One of Montoni's servants. He falls in love with Annette and provides assistance to Emily. He is more sensible than Annette, and is both brave and quick-thinking.
Cavigni, Verezzi, and Bertolini: Cavaliers and friends of Montoni. Cavigini is sly, careful, and flatteringly assiduous. Verezzi is a "man of some talent, of fiery imagination, and the slave of alternate passions. He was gay, voluptuous, and daring; yet had neither perseverance or true courage, and was meanly selfish in all his aims." Bertolini is brave, unsuspicious, merry, dissipated, and of extreme extravagance; his free flightiness to Emily distresses her.
Orsino: An assassin described as the "chief favourite of Montoni". He is cruel, suspicious, relentlessly vengeful, and merciless.
Marchioness de Villeroi: A mysterious figure whose miniature Emily discovers in a secret panel in her father's closet. She was married to the Marquis de Villeroi, but becomes estranged from him and dies thanks to the intervention of Laurentini di Udolpho. She was sister to M. St. Aubert, making her Emily's aunt.
Signora Laurentini di Udolpho (also called Sister Agnes): A nun living in the French monastery of St. Claire. She dies in the final volume of the novel, whereupon she is revealed to be Signora Laurentini, heiress of the house of Udolpho. She estranged the Marquis de Villeroi, her first love, from his wife, after which she retired to the monastery to live in guilt. She divides her fortune between Emily and the wife of M. Bonnac.
The Marquis de Villeroi: Lover of Laurentini before he married the Marchioness. He leaves the Chateau-le-Blanc after her death.
Francis Beauveau, Count De Villefort: Heir to the mansion at Chateau-le-Blanc in Languedoc. He inherits the chateau from his friend the Marquis de Villeroi. He has two children from a previous marriage, Blanche and Henri, and is married to the Countess De Villefort.
Lady Blanche: A sweet young woman who has a deep appreciation for the sublime and writes poetry. She resides at Chateau-le-Blanc and befriends Emily, with whom she shares many interests.
Henri: Blanche's brother.
Dorothée: A servant at the Chateau-le-Blanc. She is superstitious, like Annette.
Monsieur Du Pont: One of Emily's suitors. He steals a miniature of Emily belonging to her mother, which he later returns. He helps Emily and her companions escape from Udolpho. He is a friend of De Villefort, who supports his suit. When Emily steadfastly rejects him, he turns his attentions to Blanche, but is thwarted again when she marries St. Foix.
Monsieur Quesnel: Emily's uncle. He is cold and unfeeling towards Emily until she becomes an heiress.
Madame Clairval: Valancourt's aunt and an acquaintance of Madame Cheron. She initially approves of the match between Valancourt and Emily, but finally decides that there are better prospects for both of them.
Monsieur Bonnac: An officer in the French service, around fifty years old. Emily meets him at the convent. His wife inherits the castle at Udolpho.
Monsieur St. Foix: Suitor of Blanche. He marries her at the end of the novel.