Joker, or Father Joker, as he is referred to in the Church of Claudandus, is an old white cat and the leader of the Claudandus sect - a widespread cult among the cats of the city that worship Claudandus as "the Prophet". Cats take part in ceremonies that included masochistic sessions of electrocution.
Joker accidentally told his great-granddaughter Pepeline a different story about Claudandus, which made Francis to suspect him to be involved in the murders.
However, Joker disappeared, leading Pascal to declare him as the murderer of the cats. Later it is revealed that Joker helped Claudandus to carry out his plans, preparing the cats spiritually for the coming of the Felidae Breed, while Claudandus took care of the actual breeding project and the murders. Joker was assassinated by Claudandus with Joker's consent to sidetrack the investigation and to let the feline community believe he was the murderer. At the end, his body was found by Francis.
After both Joker and Claudandus die, the Felidae project is brought to its end.
In the book, the ultimate fate of Joker is never revealed by Francis to the other cats of the city, thus allowing cats to think that he had simply left to spread his cult among other cats.