Crookshanks is the cat bought by Hermoine Granger at the beginning of the book Prisoner of Azkaban. He is known to be very clever and had the ability to communicate with Sirius Black when he was in he animagus form.
Prisoner of Azkaban
Before the start of the school year in 1993, Hermione Granger purchased Crookshanks from Magical Menagerie in Diagon Alley. When buying the cat, Hermione was told that he had been there for ages and no-one wanted him. Being an intelligent cat, Crookshanks sensed that Scabbers, Ron's pet rat was not to be trusted and took an immediate dislike to the rat. The pair had several fights throughout the year and eventually Crookshanks was falsely accused by Ron Weasley of killing and eating Scabbers, who himself has a dubious reputation. Scabbers the rat was found to be the secret Animagus form of Peter Pettigrew a follower of Lord Voldermort who revealed the secret of where Harry's parents were hidden. Crookshanks was cleared when Rubeus Hagrid gave Scabbers back to Ron in his hut just before the Minister of Magic, Cornelius Fudge and an executioner arrived to behead the hippogriff Buckbeak. Crookshanks also consorted with a mangy looking dog who turned out the be the Animagus Sirius Black, Harry's god-father who had been falsely accused of murdering Peter Pettigrew and numerous muggles. For this crime Sirius had been sentenced to the Wizard's Prison at Azkaban. He met with Sirius several times on the lawns of Hogwarts castle and even stole the passwords to the Gryffindor commonroom for him.
In the books following Prisoner of Azkaban, Crookshanks has a minor role and is only seen in minor references.
Crookshanks is described as a lion like, ginger cat with a squashes together face. He also has a large bottlebrush tail.