Doreen Green was born to Dorian and Maureen Green. She suffered a modification in her genes for unknown reasons that granted her squirrel-like abilities, which manifested predominantly in the form of a prehensile tail. When her parents consulted with a doctor, it was determined that Doreen wasn't a mutant, even though she believed so for a long time. When she was ten years old, Doreen discovered she could communicate with squirrels after overhearing one of these rodents in her window. She subsequently saved this squirrel from being chased by a dog, and they became friends. The squirrel, who identified himself as Monkey Joe, encouraged Doreen to use her abilities to help people. Ever since then, Doreen began to fantasize about becoming a superhero, and came up with the alias of the "Unbeatable Squirrel Girl."