Mittens is a sassy and short-tempered street cat who Bolt at first believed was an agent of Dr. Calico, but later they became friends. She is pessimistic and has a dry sense of humor. She is a thief and a survivor. She wasn't interested in friendship at first, but she is eventually moved by Bolt's loyalty. Judging by her color patterns, she appears to be a Tuxedo cat. She speaks in a New Yorker accent.
Mittens has a very sad background as she was originally a house cat who was owned by a seemingly loving family. However, her owners moved away and left her to brave the harsh streets of New York City alone and declawed, which would have decreased her chances of survival due to her lack of claws. Being abandoned got her to adopt a strong distrust and hatred of humans and a "look out for #1" attitude. She managed to survive in New York by gaining control of several pigeons who fear her by forming a mafia-like organization and forcing the pigeons to bring food for her in exchange for protection, however if a pigeon fails to bring enough food to her (Take Louie for example, a pigeon who only brought Mittens half of a crumb) she threatens to use her claws on them and eat them even though she is declawed and therefore has no claws (however the pigeons are unaware of that fact), therefore she is bluffing. She got her just desserts when three pigeons who also worked for her named Joey, Vinnie, and Bobby tricked a disillusioned dog named Bolt into believing she'd aided in kidnapping Bolt's owner, Penny, in order to put an end to all the misery they've been brought up to by Mittens. She plays along with Bolt's mistaken beliefs and eventually teaches him how to beg for food with something called "the dogface". They meet a TV-obsessed hamster named Rhino in a trailer park who joins them, and Rhino makes a comment about a "magic box", that enables Mittens to figure out Bolt is part of a TV show, but before she can properly explain, she and Bolt are captured by animal control, but Bolt escapes. Mittens ends up at the pound and loses hope, but is granted a miracle in Bolt coming to her rescue, admitting that he has no super powers. They escape the pound together and continue on their journey while Mittens shows Bolt how to act like a real dog.
When they reach Las Vegas, Mittens finds it a goldmine of food and comfort, and tries to convince Bolt to stick around and that none of what he knew back at the studio is real, including that Penny, being an actress, doesn't love him. Bolt refuses to believe that, causing Mittens to rant and reveal her sad past to Bolt. Bolt insists in believing Penny loves him and sadly bids Mittens farewell, and continues on to Hollywood.
Mittens, feeling depressed, explains to Rhino that Bolt left to "face the green-eyed man alone". Rhino convinces Mittens they can't let Bolt go off alone and they follow him to Hollywood. Mittens witnesses the rehearsal between Penny and a Bolt lookalike but so does Bolt, who leaves, thinking the tender moment he saw between them was real. But Mittens sees that the tender moment between Penny and the Bolt lookalike was just acting and that Penny really does miss Bolt and rushes to tell him. Bolt senses Penny is in danger and they rush to the studio, which has caught on fire. Bolt manages to save Penny and himself. As Bolt and Penny are taken to an ambulance, Mittens and Rhino stow aboard and are eventually adopted by Penny and they all move to a rural home, and Mittens finally gets the family she deserved. She also learned that not all humans are bad after all.