The cat is one of the few animals that never work and she is intelligent enough to see corruption.
In the novel, she is seen during Old Major's speech. During the 'are rats comrades'-voting, she actually votes for both sides. She never works, but her excuses are so excellent and she purrs so affectionately it is impossible not to believe in her good intentions. She has no interest in the politics on the farm. But finally, she leaves the farm, avoiding any conflict.
In the 1954 animation, she has a lesser role and is killed by two of Napoleon's dogs.
In the movie of 1999, the cat is never mentioned.
It's a bit unclear who the cat represents, but she likely represents the 'shady' people of Russia like thieves and gypsies.