Crookshanks is a large, fluffy, ginger cat, who has lived in the Magical Menagerie for a long time. He is bow-legged and has a grumpy, squashed face. When Ron, Hermione and Harry are in the shop, Crookshanks, spitting wildly, leaps onto Ron's head and attempts to attack Scabbers before being bought by Hermione as a pet.
Once on the Hogwarts Express, Hermione lets Crookshanks out of his basket. The cat springs onto Ron's knees looking for Scabbers. He spends the majority of the journey on an empty seat, eyeing Ron's top pocket, where Scabbers is hiding.
Crookshanks slinks towards the trio as they make their way back to the castle. He chases after Scabbers when the rat escapes Ron's grasp, but is unable to catch the rat before Ron.
After Ron and Scabbers are dragged into the tunnel beneath the Whomping Willow, Crookshanks darts between the branches of the tree and places his paws on a knot on the trunk, which freezes its movements. He slides into the gap at the roots of the tree, followed by Harry and Hermione.
Crookshanks leads Harry and Hermione along the tunnel. As the passage ends, Crookshanks disappears from sight. He goes into the room at the top of the stairs, and is found settled in the middle of the four-poster bed when Harry and Hermione enter. During the fight between Harry and Sirius Black, Crookshanks claws Harry in the arm, and tries to get at Harry's wand on the floor. He moves when Harry tries to kick him aside, spitting furiously. The cat leaps onto Black, who is lying on the floor, and settles on his chest. Crookshanks refuses to move when Black tries to push him off, sinking his claws into Black's robes to help him stay put. Crookshanks only moves when Scabbers is removed from Ron's pocket; he stands on Black's lap and hisses at the rat.
Black tells Harry that Crookshanks is the most intelligent cat he has ever met. He tells them that Crookshanks recognised Pettigrew for what he was immediately, and that he knew Black was no dog when they met. It took a while before Crookshanks trusted Black, but he finally managed to communicate what he wanted to the cat. Crookshanks is the one who has been helping Black, initially trying to bring Pettigrew to him. When that failed, Crookshanks stole the passwords for Gryffindor Tower from Neville Longbottom's bedside table and brought them to Black.
Crookshanks leaps lightly off the bed when the group prepare to leave the Shrieking Shack and leads the way out of the room, his tail held jauntily high.
Crookshanks leads the party out of the Shrieking Shack and back through the tunnel. He darts through the entrance at the Whomping Willow and presses his paw to the knot at the base of the tree, freezing its movements so the rest of the group can climb out safely.
Crookshanks backs away, fur on end, when Lupin starts to transform into a werewolf. He is blasted into the air by Pettigrew during his escape, and lands in a heap on the ground.
Crookshanks sits on a seat in the train compartment as they journey to London. When Sirius's letter is delivered, Crookshanks sits up in his seat, following the tiny owl who delivered it with beady eyes. Crookshanks purrs when Ron holds out the owl for him to inspect, which confirms for Ron that there is nothing suspicious about the animal.
Crookshanks wanders over to Ron and Harry in the Gryffindor common room. Leaping lightly into an empty chair, he stares inscrutibly at the two boys, rather like Hermione would if she knew they weren't doing their homework properly. Crookshanks curls up in Hermione's lap when she joins Ron and Harry in the common room.
Crookshanks squirms in Ginny's arms as he is carried from the Hogwarts Express at Hogsmeade station. He is passed to Hermione, who carries him into one of the Hogwarts carriages, ready for the journey up to Hogwarts.