Shirley, known in Japan and Tag Force as Karen, is the crocodile that Jim Cook carries around on his back. She is considered by Jim to be family and he carries her around in a special backpack. He often uses her as a judge of character when meeting someone new, which led to his friendship with Jaden Yuki. Jim met Shirley while walking through a forest as a kid, when he spotted her unknowingly heading towards a trap. He jumped towards her and pushed her out of the way, saving her from injury. They have been together ever since.

During the school's opening ceremony, when Jim shakes hands with Jesse Anderson, Shirley growls at Jesse and he freaks out. When Jim first meets Hassleberry, Shirley attacks him and Jim puts it down to the electro-magnetic waves emitting from Viper's lab. When Jim is defeated by The Supreme King, Shirley attempts to block the blow but is killed with Jim. She later comes back with everyone else.