Ancillary franchise member. Squirrel Girl has been a member of the spin-off rosters of Great Lakes Avengers (GLA #1) and A.I.M.'s Avengers (New Avengers (4th series) #1). She was also an in-house associate for the Avengers as Danielle Cage-Jones's nanny as of New Avengers (2nd series) #7.
Squirrel Girl was originally believed to be a mutant, but an official note from her doctor confirmed otherwise in Unbeatable Squirrel Girl (2nd series) #1.