Comic book superhero, published and owned by
Marvel Comics. She was created by
Steve Ditko and
Will Murray in "
Marvel Super-Heroes #8" in 1992.
Squirrel Girl's real name is Doreen Green, and she's a mutant with a variety of squirrel-related powers. She has a large, furry, prehensile tail, buck teeth strong enough to chew through wood, claws and retractible knuckle-spikes, enhanced strength and agility, a strong sense of smell, and the ability to communicate with squirrels. She has a utility belt of pouches which she uses mainly as a stash for nuts to give to squirrels; she calls these her "nut sacks." She also claims that her lips taste like hazelnuts, and she has a relentlessly upbeat and positive outlook on life and superheroing.
Doesn't sound like much, right? Wrong. Squirrel Girl is the biggest badass in the Marvel Universe.
In her first appearance, SG ambushed Iron Man, hoping to become his sidekick. When the megalomaniacal supervillain and dictator Dr. Doom shows up and captures Iron Man, the 14-year-old mutant causes scores of squirrels to swarm Doom's plane, leading to what was probably Doom's most humiliating defeat.
After that, Squirrel Girl made no appearances in any Marvel Comics for a decade. Finally, in 2005, writer Dan Slott wrote a miniseries about the Great Lakes Avengers (later calling themselves the Great Lakes Initiative, or GLI), a team of weirdo superheroes who included Flatman, Doorman, Big Bertha, and Immortal Man, who tended to die and come back to life several times every issue. Slott decided to make Squirrel Girl a new member -- in fact, she introduced and made commentary during every issue.
During the course of the miniseries and a number of follow-up one-shots, Squirrel Girl has defeated MODOK, Terrax, Thanos (Earth's Watcher, Uatu, made an appearance just to confirm that yes, it really was Thanos, not a clone or a copy or robot duplicate or something), the Mandarin, Deadpool (who's been spanked by her several times and considers her one of the powerhouses of Marvel's superheroes), Maelstrom, Giganto, and Bi-Beast (She beat him during a team-up with the Thing -- she got her squirrels to pelt the Thing and Bi-Beast with incredibly smelly garbage while they were fighting. Bi-Beast had to use both hands to cover both his noses, and the Thing cleaned the villain's clock.) She once waltzed into Doctor Doom's castle in Latveria -- and Doom just let her go about her business, because he was afraid to confront her.
Sure, Squirrel Girl seems to be a joke character, and her many victories could just be a gag about low-powered characters who improbably defeat really powerful villains. But as Tippy-Toe said in the "Deadpool-GLI Summer Fun Spectacular" from 2007:
"Yeah, that's right. Squirrel Girl totally pwns Doc Doom. Know why? 'Cause of somethin' that happened in a story by Steve-freakin'-Ditko! That's so in continuity. So just deal with it, fanboy."
For a long time, Squirrel Girl was supposed to be madly in love with the hero Speedball, and she was very unhappy about his transformation into masochistic emo-boy Penance. Her primary and most intelligent squirrel buddies include Monkey Joe (recently deceased) and Tippy-Toe. At one time, she was offered a position with S.H.I.E.L.D. because of her uncanny abilities at beating up supervillains, but she preferred hanging out with the Great Lakes Initiative.
Starting in 2014, Squirrel Girl enjoyed a much more prominent place in the Marvel Universe. She got her first ongoing comic, called "The Unbeatable Squirrel Girl," written by Ryan North (of "Dinosaur Comics" fame) and illustrated by Erica Henderson. The series focused on the comedic adventures of Doreen as she studies computer science at Empire State University. Her allies included her roommate, Nancy Whitehead, and two animal-controlling superheroes, Chipmunk Hunk and Koi Boi, along with occasional assistance from Howard the Duck, Kraven the Hunter, and Iron Man (though she mostly spars with Tony Stark on Twitter). She battles foes like Galactus, Venom, the Mole Man, Whiplash, Ratatoskr, Nightmare, and Dr. Doom, often defeating them with cleverness and empathy rather than power -- she talks to and befriends Galactus and helps him find a better planet to devour than Earth (no life other than delicious, nutritious nuts!) and she beats Nightmare using, of all things, binary hand code.
Only a few years later, she got her first graphic novel, "The Unbeatable Squirrel Girl Beats Up the Marvel Universe," which is pretty much what happens, as Doreen's not-exactly-evil-but-much-less-good clone beats up the Marvel Universe, forcing the real Doreen to figure out a way to beat the unbeatable. She's also joined a couple different Avengers teams, often providing a little comic relief along with the inevitable ass-whuppins.
Research: Mostly from The Appendix to the Handbook of the Marvel Universe