Flush is a well-bred cocker spaniel with a dark brown coat, hazel eyes, and tasseled ears. Born in the countryside, he initially enjoys running freely until he relocates to a wealthy Wimpole Street neighborhood in London, where the sensory overload frightens him. He possesses a highly developed sense of self and empathy, choosing to suppress his desire for outdoor exercise to remain constantly at the side of his chronically ill owner.

Elizabeth is a celebrated Victorian poet who suffers from a chronic illness that confines her to her room in her family's house. Supported by her father's merchant wealth, she spends her days reading, writing, and receiving occasional visitors while relying on laudanum for pain management. When she receives a young spaniel as a gift, she forms an immediate, empathetic bond with the animal.

Robert is a rising literary figure who reads Elizabeth's published poems and initiates a passionate correspondence with her. He frequently visits the Wimpole Street house, bringing energy and urgency into the quiet environment. His growing connection with Elizabeth initially angers her dog, but he remains undeterred and eventually brokers a tentative peace.

Wilson is a working-class maid employed by the wealthy Barrett family to attend to Elizabeth's daily needs. She maintains the cleanliness of the room, runs errands in the city, and provides practical support. She often takes charge of walking the family spaniel and enforces discipline when necessary.

Mary is a writer who lives in a country cottage and originally breeds Flush. Although she and her husband experience severe financial difficulties, she refuses lucrative offers to sell the young, well-bred spaniel. Believing the dog will provide comfort, she chooses instead to gift him to her invalid friend in London.

Mr. Barrett is a wealthy merchant and the patriarch of the Wimpole Street household. He exercises intense control over his twelve children, demanding strict obedience and actively discouraging their independence. He refuses to negotiate with criminals, prioritizing his principles over his daughter's distress when her pet is taken.

Henry is one of Elizabeth's many brothers living in the Wimpole Street house. Caught between his sister's desperation to recover her kidnapped dog and their father's strict refusal to pay extortionists, he acts as an uneasy intermediary with the criminal underworld.

Albert is another of Elizabeth's brothers. When criminals attempt to negotiate a ransom for the family's stolen dog, he reacts with hostility, refusing to be swindled and directly confronting the extortionist despite the risk to the pet.

Septimus is one of Elizabeth's brothers. Recognizing his sister's intense distress over her kidnapped dog and the failure of previous negotiations, he breaks the stalemate by offering to go into Whitechapel and pay the demanded ransom himself.

Mr. Taylor is the alleged leader of a dog-kidnapping ring in London. He preys on the attachments of wealthy pet owners, stealing beloved animals from affluent neighborhoods and holding them in squalid conditions until ransoms are paid.

Signor Righi is an Italian guardsman who crosses paths with the family after they relocate to Florence. He becomes a brief romantic interest for the family's maid, representing the new social opportunities available in the freer Italian environment.

Robert Wiedeman Barrett Browning, affectionately called Pen, is the young son born to the family in Italy. His arrival initially confuses the household pet, but he grows up happily playing in the nursery during the family's most peaceful years abroad.