Duke is an enormous and overbearing brown Mutt. He has long hair, big legs and a black leather collar with a letter "D" on it (the collar is later robbed by Ozone).
Duke is sloppy and oafish. He doesn't share the bed with Max and eats from his bowl. He is at first reckless and inconsiderate, but is later shown to have a sympathetic background.
BioWhen Duke was a young puppy, he had a owner, named Fred, but he ran off chasing a butterfly and he could not find his way back, which has resulted in the Animal Control putting him in custody. One day, Katie adopted him from the pound. Although Katie introduces Duke to Max as his new brother, Max does not want to live with a new dog; Duke licks Max even if Max snarls at him, which makes Max disgusted.
Max becomes more upset about how Duke behaves. Duke accidentally breaks a ball Buddy found, which Katie thought it's their missing ball. When they are about to sleep, Duke sleeps where Max is supposed to sleep and snores. The next morning, Chloe advises him that he needs to do more or else his life would be ruined. On the other hand, Duke eats Max's dog food; so Max begins to threaten Duke by breaking things in the apartment and later treats Duke as a servant when he, Buddy, Mel and several other dogs are taken to the dog park.
This drives Duke to decieve Max into downtown New York, while other dogs are distracted by a butterfly. He runs into a street where Ozone, the leader of a cat gang called The Alley Cats, are at. At first, he tries to abandon Max there but they both are defeated by Ozone and numerous cats. Their collars are stolen, which causes them to get captured by two Animal Control officers.
Nevertheless, Snowball, the leader of The Flushed Pets, appears and saves Ripper, who is also caught by the Animal Control, and Max and Duke persuades Snowball to unlock their cage by claiming they are not domestic dogs. Snowball is convinced and takes them to his underground lair. There, Snowball wants the two dogs to tell the story about "how they killed their masters" so Duke and Max makes up one, then Snowball orders a giant bushmaster called Viper and wants him to bite Max and then Duke as a sign of being a new member. But later the identity of Duke's and Max's are exposed by the Alley Cats, so all animals of The Flushed Pets begin to attack Max and Duke.
Duke bites Viper by tail and swipe the approaching animals and throws it to a pipe. Viper attempts to attack Max, but he is too long and is tied to the pipe, and his big move to escape causes loads of bricks and concrete crashes on him, so he dies instantly. Then, Duke and Max run away from Snowball's chase and escape the sewer; they get into a river and go on to a boat. Yet, the boat is going to Brooklyn, the opposite site of their home. When they reach the bank, they want to find something to eat but unsuccessful. They sit in front of a man who is eating a sandwich, they try to do the "puppy eyes" but the man runs away in horror. Duke then notices something and sniffs; he runs with Max and they find a sausage factory, Weiner Kingdom, so they gorge on sausages.
Duke later tells Max that he had another owner called Fred, but Duke can't find the way home after he finally eats a butterfly he chased, so he was sent to the pound. Max encourages Duke to find Fred, and when they are about to do so, the two Animal Control officers enters and want to catch them again but they are crashed by several boxes of sausage after Duke kicks on it. Duke finds the house Fred lives, but a cat called Reginald is there and tells him and Max that Fred was dead and the house now belongs to a new family. Resentful and angry, Duke accuses Max of attempting to get rid of him and barks at the new homeowners, who just return to their "own" house, who call Animal Control. The Animal Control officers suddenly appears with a new truck and catch Max, but Duke interferes long enough for him to escape and ends up being captured instead. Max chases after the truck.
On the other hand, Snowball and other animals arrives to Brooklyn; he disguises as a baby and Tattoo as a woman, and they find Max, but other animals are caught except Snowball and Max. So Snowball and Max drive a bus and stops the Animal Control's truck, but the truck is almost falling to the river and hangs on Brooklyn Bridge. Max finds the key to the cage but the truck falls into the river and the key is dropped in the process, so Snowball also jumps into the river, finds the floating key and gives it to Max. Later, Tattoo drives a taxi and brings all animals to Manhattan. At last, Duke and Max go back to their apartment in the night before Katie goes home.
- "Don't make me angry, because when I get angry, I do this...(growls)... and I hate doing that."
—Duke threatens Ozone
- "Mind your own business...Oh my gosh, what happened to you!"
—when Duke first sees Ozone's horrible look
- "Get your umbrellas out, kitties, because here they come! The Thunder, and the lightning! Right down on your face!"
—After Ozone takes away his collar
- "If I had a dime for every owner I killed I would have a dime."
—Duke obviously lying to convince Snowball that he should be killed
- "You're doing...Well, you're not doing great. But you're not drowning, and that's something."
—To Max who is struggling to grab the ring behind the ferry in the swift current
- In The Secret Life of Pets, it is never revealed about the previous name that Fred gave to Duke. Nevertheless, Duke's smaller collar with a sign "D" can be briefly seen when he was a puppy, and the bigger collar was also given by Fred, which may give Katie a reference of giving his name.
- Duke is similar to Buzz Lightyear from Toy Story. Aside from being the satellite to the main characters, Max and Woody, they are bigger in life than their own friends. They also started the plot of being lost from their owners and in the end they get in terms with their own friends. He also carries similarities to Jessie in that they have former owners, the difference is that Jessie is left due to her owner being grown up while Duke's owner implied to be dead by the time he's in the pound.
|Buddy • Chloe • Duke • Gidget • Max • Mel • Norman • Pops • Sweet Pea • Tiberius • Daisy • Hu • Rooster|
|The Flushed Pets|
|Derick • Dragon • Nitro • Ozone • Password Snakes • Ricky † • Ripper • Snowball • Tattoo • The Alley Cats • The Sea-Monkeys • Viper †|
|Dummy • Gino • Guillermo's Iguana • Leonard • Marilyn's Cat • Myron • Peanut • Pepe • Pickles • Reginald • Runty • Shelly • The Boston Terrier • The Collie • The Golden Retriever • The Papillon • Tiny|
|Fernando • Maria • The Monkey • The Piranha • The Sausages (Frank • Timmy) • The Squirrels • The Pigeons • The Wolves • Unnamed Chihuahua|
|Animal Control (Fourrage • Menard) • Buddy's Owners • Chloe's Owner • Fred † • Harold • Gidget's Owners • Gino's Owners • Guillermo • Guillermo's Girlfriend • Katie • Katie's Grandma • Leonard's Owner • Marilyn • Mel's Owner • Molly • Norman's Owner • Pepe's Owner • Reginald's Owners • Sergei • Steve Cabin • Sweet Pea's Owner • Tiberius' Owner|