Lily is in a band with Larry called The Geeks, in which she is lead singer. While practicing for their school's Battle of the Bands, Larry comes across a bottle of Insta-Tan. Larry, Lily, and their friends Manny, Jared, and Kristina apply it, hoping it will give them a great tan. Larry is shocked to discover the bottle expired one year prior, but Lily assures him there's no danger. As the date of the Battle of the Bands approaches, Larry begins to act strange. His body becomes covered with thick, black hair that constantly grows back after he shaves it. He asks Lily if she's been experiencing the same thing and she teases him. Later, their friend Manny disappears. Lily and Larry discover his house completely bare. The two don't have an answer as to why he moved in such a hurry.
Larry's problem continues. He decides to ask his friends again if they have been growing hair. This time, Lily seems frightened and quiet. Larry tries to get answers, but Lily is hesitant to reveal anything. Eventually, Lily vanishes like Manny did. Larry tries to call her, but there is no answer. Eventually, he comes across a dog that wears Lily's gold pirate coin around her neck. Even stranger, it has two different eye colors like Lily. Larry realizes the dog is Lily, and goes to her parent's house to question them. They tell Larry there is no Lily, and to forget about her.
Lily was part of an experiement by Larry's doctor, Dr. Murkin. He created a serum that turned dogs into human children. However, it wasn't able to last long on dogs and the children quickly reverted to their dog forms. Now dogs, Lily and Larry play together.
Lily is snarky, and loves to tease her friend, Larry. She calls him "Hairy Larry" due to his obsession with making sure his hair looks good. Lily is known to write long poems about nature, which makes her more appreciative of her surroundings. When Larry begins to act strange, she shows concern for him, as it is likely she is experiencing the same thing. When she turns into a dog, she tries her best to tell Larry, but is unable to communicate with him. When Larry is reverted to his dog form, the two seem to continue their friendship, albeit in a much different manner than before.
Lily is described by Larry as short and cute, with chin-length blonde hair. Her predominant feature is her two different colored eyes, one blue and one green. In dog form, she has light brown fur, almost red, with a white patch on her throat. She retains her two eye colors. Lily wears a gold pirate coin around her neck that was given to her by her grandfather.
- In the television adaptation of the book, Lily's surname is changed from Vonn to Turnbull.
- The condition that gives Lily her two different eye colors may be heterochromia iridum, which is more common in animals.