Duchess is very elegant, beautiful and ladylike, being the pet to an aristocrat. She is very devoted to her owner, Madame Adelaide Bonfamille. She is kind and friendly to everyone. She is both caring and protective toward her children and also tries to encourage them to develop into proper members of French society. She also is a connoisseur of music and presumably teaches her kittens to sing and play piano.
Duchess also is shown to be slightly pacifistic, as she scolds her kittens for "play-fighting". She is sweet and compassionate and is willing and able to make friends with anyone, no matter their background. She also seems to show concern for others, as is evidenced when she calls out to Thomas O'Malley as he floats down the river after he throws Marie to safety.
Duchess is a long-haired purebred white Turkish Angora cat with blue eyes later described as shining like sapphires. She wears a gold, jeweled collar studded with diamonds.
Duchess lives with her owner and retired opera singer, Madame Bonfamille, as well as her three kittens: Marie, Berlioz, and Toulouse, in a large mansion in 1910 Paris. At the beginning of the film, the family is returning from a carriage ride. Back home, Duchess oversees Toulouse's painting lesson and Marie and Berlioz's music lesson. Later, Madame Bonfamille meets with her lawyer and dear friend, Georges Hautecourt. After kissing Duchess's tail, (which the lawyer believes to be Madame Bonfamille's hand) they settle down to business. Madame reveals that, after her death, she intends to leave her vast fortune to Duchess and her kittens. At the end of their lifespan, Madame's butler, Edgar would inherit. Edgar overhears, and plots to remove the cats from the inheritance, as he believes he would die before getting to inherit.
That night, Duchess and the kittens are drugged using milk mixed with sleeping pills. Edgar catnaps Duchess and her kittens and leaves them in the French countryside, so that Madame Adelaide would think the cats had decayed so that she could give all her fortuns to Edgar. The next morning, Duchess meets Thomas O'Malley, an alley cat, who offers to guide them back to Paris. They have a struggle returning to the city, briefly hitchhiking on the back of a milk cart before being chased off by the driver. Marie subsequently falls into a river and is saved by Thomas. They then meet a pair of English geese, Abigail and Amelia Gabble, who are travelling for Paris. The group head off, marching like geese, until they reach Paris and come across the girls' drunken Uncle Waldo. Abigail and Amelia then depart to take Waldo home.
Traveling across the rooftops of the city, the cats meet Scat Cat and his band, close friends to O'Malley, who perform the song Everybody Wants to Be a Cat. After the band has departed and the kittens lie in bed, O'Malley and Duchess spend the evening on a nearby rooftop and talk, while the kittens listen at a windowsill. O'Malley offers to have Duchess stay and be with him, but she sadly turns him down, largely out of loyalty to Madame Adelaide.
The next morning, Duchess and the kittens make it back to the mansion, whereupon O'Malley departs sadly. Edgar sees Duchess and kittens coming and captures them, placing them in a sack hidden in an oven. Duchess tells Roquefort to pursue O'Malley and get help. He does so, whereupon O'Malley races back to the mansion, ordering Roquefort to find Scat Cat and his gang. Edgar places the cats in a trunk which he plans to send to Timbuktu, Africa. O'Malley, Scat Cat and his gang of Alley Cats, and Frou Frou all fight Edgar, while Roquefort frees Duchess and the kittens. In the end, Edgar is tipped into the trunk, locked inside, and sent to Timbuktu himself. Madame Adelaide's will is rewritten to exclude Edgar and include O'Malley and any future kittens he and Duchess have. Madame also starts a charity foundation providing a home for all of Paris's stray cats. The grand opening thereof, to which most of the major characters come, features Scat Cat's band, who perform a reprise of Everybody Wants to Be a Cat.
Duchess made a few cameo appearances in the animated series. Her only speaking role was in "Ladies' Night" where Daisy Duck announced a phone call for Duchess, having many Disney Princesses believing she's referring to them. Duchess then arrives and states, "Darlings, she said, 'Duchess,' not 'princess,'" and picks up the phone. This was also the only time in which a character that was previously voiced by Eva Gabor has dialogue.
- She is voiced by the late Eva Gabor, who also voices Miss Bianca in The Rescuers.
- Some of Duchess' dance moves were infamously reused for that of Maid Marian's in Robin Hood.
- Duchess bears some resemblance to Mewsette from the Warner Bros/UPA film, Gay Purr-ee, which coincidentally also featured talking cats in Paris, France.
- It's possible that after marrying Thomas O'Malley, Duchess' full name now appears to be Duchess Bonfamille-O'Malley.
- It's likely that Duchess's first mate (the father of her litter) had either orange and/or black fur, due to Toulouse and Berlioz having those fur colors.
- Her name means wife or woman of a duke who has the same equivalent rights to be a monarch ruling over a duchy or a member of the nobility.
- After she and her kids are trapped by Edgar, Duchess has no more lines but can be heard laughing at Toulouse as he taunts Edgar who is being taken to Timbuktu for his punishment.