Lilly is a small brown puppy with lighter brown chest and a dark-spotted eye. She also wears a pink bow around her neck.
Lilly is first seen in the episode Cookie Chomper III where Dave Seville takes the Chipmunks to an animal shelter to pick up a new pet after the death of their cat, Cookie Chomper III. At first the boys are reluctant to "replace" Cookie Chomper III but quickly take a liking to Lilly and adopt her into their home. From this point on Lilly appears briefly in a few subsequent episodes as the household pet, shown to be affectionate to the boys as well as causing trouble such as drinking Simon's hair formula.
She appears as a main character in Three Chipmunks and a Puppy where the boys, frustrated with their attempts to train her, decide to give her away. After seeing some videos of Dave's attempts to train them when they were younger, they decide to accept responsibility and adopt Lilly again.