The Narrator is Kitty as an adult.
Kitty eventually outgrew her cat costume, and this was the moment where she began to finally accept the fact that she was a human, not a cat. She soon started talking normally, slowly stopped acting like a cat, and eventually went to school; much the mixture of joy, sadness, regret and pride of the cats, as they had grown fond of how cute she was. However, she still held onto her love of cats, and continued to live in the mansion with her loving feline family with bonus of being able to communicate normally with them. As she reached her late teens, Kitty realized that the cats' inheritance would run out soon, seeing as none of the cats had jobs, thus leaving them with no way to pay their bills. Not only that, she was very much aware that neither the cats nor herself wanted to see her go. So, she stayed in the mansion as an adult and became a successful author to ensure a comfortable life for everyone. The books that she wrote: a series of children's stories called 'Kitty Is (Not) a Cat', which is actually her own autobiography, unbeknownst to the public. The entire cartoon is either just Kitty doing a reading of her books, or an in-universe animated series based on her books. Alternatively, the Narrator is the eccentric baroness that left the mansion to the cats.
She still keeps an eye on her old mansion and everyone inside, and has grown quite fond of Kitty. She is either watching from above and telling stories to children in Heaven, or her spirit still resides within the house and is either talking to herself or is somehow in contact with someone via supernatural means for the sole purpose of being a storyteller. Kitty is autistic.
It would explain her obsession with cats, her less than stellar social skills, and her surprising level of intelligence as shown in "School For Cats" where she solves a Rubik's Cube in record time and in "The Playhouse" where she's able to decipher a complex set of blueprints in order to construct the titular playhouse all by herself. The show takes place in the same universe as Foster's Home for Imaginary Friends
The animation style, humor, character design, and tone is very similar to start with. It would also explain why the cats are not only anthropomorphic, but also so much bigger than real cats. The cats might actually be imaginary friends themselves, who belonged to a member of the eccentric baroness' family, if not the baroness herself. With a whole mansion all to themselves, they had no need to go to Foster's. Kitty grows up to be a feline zoologist.
It's made pretty obvious by the episode "All About the Bass" that canonically (whether shown in the show or not) The Nazz might eventually have to convince the other cats to let Kitty go once she's old enough to live her life as a human. Perhaps the times she's spent with the cats will inspire her to study other felines for a job in the future (or in a darker scenerio, become a Crazy Cat Lady trying hard to relive her childhood). In the final episode, Kitty will tell the story of her life to a girl with pale skin and multicolored hair
And when she's finished, the other girl will say "So you were raised by cats, huh? Let me tell you about my family"- Jossed; The show ended already.
