Joker (or Father Joker, as he is referred to in the Church of Claudandus) is a white old cat and he's the leader of the Claudandus sect- a widespread cult among the cats of the city that worship Claudandus as "the Prophet". Cats take part at ceremonies that included masochistic sessions of electrocution.
The little Pepeline is his great granddaughter and Joker accidentally told her a different story about Claudandus.
During the story, he disappeared leading Pascal to declare him as the murderer of the cats. Later it is revealed that Joker helped Claudandus to realize his plans and was assassinated by him 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.
In the novel it is also revealed how Joker was preparing for cats on a spiritual point of view for the coming of the Felidae Breed, while Claudandus took care of the actual breeding project (and the murdering).
Trivia:. Both him and Claudandus (the two antagonists of the story) die, leaving the Felidae project to its end.
The ultimate fate of Joker, in the book, will never be revealed by Francis to the other cats of the city. Cats will think that he had simply left to spread his cult among other cats.