Crookshanks is Hermione's ginger cat with a face that looks like he ran straight into a brick wall and first comes in book three. Eventually he befriends Sirius in his dog form who begins to communicate with him, Crookshanks instantly knows that Scabbers, Ron's rat, is really an Animagus by the name of Peter Pettigrew who is the one who gave the Potters to Voldemort and blew up half a street of muggles to frame Sirius and chopped of his finger which people believed to be the biggest remaining part of him. When Harry first met Sirius and beleived that he helped Voldemort kill his parents and was going to kill Sirius, Crookshanks stood in the way so he could save him.