Doreen Green

Doreen Green is an upbeat and hyperactive mutant who dreams of one day becoming Tony Stark's sidekick. She has some very unique characteristics, including a long, bushy squirrel tail, and is always seen with her sidekick squirrel companion, Tippy-Toe.

After Meeting Mr. Man
(Doreen's background is tweeked from the canon.)

During her first meeting with Ironman, Doreen had made a promise to him. If she wanted to become his sidekick, she would first have to finish school, regardless of the boys who bullied her for her appearance. So, with a new determination, her and her first companion, Monkey-Joe, did their very best to power through and graduate. But things went from bad to worse.

She was still picked on relentlessly, now being teased for being a liar about meeting Ironman and defeating Dr. Doom. The bullying got so bad that, one day, the boys who teased her managed to get Monkey-Joe from her. Doreen cried and tried to get her friend back, but one thing led to another and things got out of hand fast. Though it was an accident, the boys ended up snapping her companion's neck, swiftly killing him. After that, Doreen left school and home and took off to the wild wilderness.

She lived in the forest for quite awhile before picking up a new companion, Tippy-Toe. With her new friend, she had her own adventures, joining a group that called themselves the Great Lakes Avengers. But, ultimately feeling like she was outshining the group, she left the team and took to New York in hopes of starting over again and trying once more to become Ironman, aka Mr. Man's sidekick.

Powers
Squirrel Girl is a mutant,  and evinces a variety of mutations which align with the squirrel theme. During her first encounter with Iron Man, she provided a detailed demonstration of her powers and abilities: a furry  tail roughly 3–4 feet in length; sizable buck teeth, which are strong enough to chew through wood; and enhanced agility and strength, enabling her to jump between trees with ease. Her fingers have sharp claws on them, assisting her with climbing, and she possesses retractable "knuckle spikes" roughly 2-3 inches in length on each hand. Most importantly, she is capable of communicating with and understanding squirrels.

Later appearances have revealed additional abilities possessed by Squirrel Girl including heightened reflexes (which she dubs as 'squirrel agility') and vision (her eyes have been seen to glow red in low-light situations) and she has also at times displayed an enhanced sense of smell. Squirrel Girl carries a utility belt  comprising multiple pouches that contain nuts to give as snacks to her squirrel friends. These are known, to much comedic effect, as her "nut sacks".