How to unlock Squirrel Girl? | FAQ - Frequently Asked Questions LEGO Marvel's Avengers Guide

Squirrel Girl is one of the most important characters in the game. Thanks to her you can dig up various items from the ground that are needed for solving riddles. She will also allow you to walk through special tunnels. The second character that is able to walk through the tunnels is Reptile whose token can be found on Manhattan, in the southern part of the central park.

Squirrel Girl can be unlocked on Barton's Farm. You must complete the Going Nutso mission which description can be found in the chapter Barton's Farm - secrets.