Sanity Cat is a kitten adopted by The Chef and Lindsay, ocassionally appearing in FNaF RPs tagging alongside the two. Her first real appearance in a game was in Human Vs. Machine, as the Cat class. She appears as a white, fluffy Persian cat with blue eyes, with a collar around her neck emblazoned with "Sanity-Freddy Fazbear's, LK."
A night guard who's name is long forgotten brought his cat to work, to help him calm down. He completed the night, but he nearly died, thus he ran out because he was scared beyond his wits, and he left the cat. Shortly after, Amity was cleaning the office when she found the cat; she gave it to Lindsay, and Chef and Linds took care of the cat together.
During the events of Human Vs. Machine, Sanity came along with Lindsay to meet the hunters; she was left there on accident, and Mitchell Cutter got a strange idea to put machine guns on the cat. Even stranger, it worked, and Sanity became a helpful asset to the hunters, boosting morale and firepower.
Sanity Cat appears in an upcoming game, as a sanity-restoring asset. Ocassionally, between checking cameras, shutting doors, you can use the cat to reduce chance of harmful hallucinations. Having ventilation working along with Sanity Cat, and almost no hallucinations occur. Another helpful ability is if an animatronic is in the office, Sanity will run off, telling you someone is near. She comes back after an animatronic has been gone for about 15 seconds.
Sanity Cat can't speak or talk, but she seems very smart and almost human-like; she has more intelligence than the average cat. She seems very attention-attracted; She appears wherever there is people.
Mitch Cutter-She seems to like being around him; Everyone says it's because of his smell of fish, or maybe Sanity trys to restore Mitch's sanity, but the two get along all the same.
Fozie-She hisses at him all the time; Most likely, Sanity has heard Lindsay and him arguiung sometimes and got the wrong impression of him.
Mangled Emy and Emy the Guardian Cat-She is very sweet towards them, as they are both nice and devoted to Sanity.
-Can sometimes be seen in the Kitchen cabinet, behind some pots and pans.
-Similiar to the Paper Plate doll, Sanity can appear on your desk sometimes. This has no effect on gameplay.
-So far, she has only appeared in HvM, as a playable character.
-Sanity first appeared when Lindsay found her in a RP; Her name at the time was "Kerfuffle". Her rename was added when she figured out about the cat's past.
-The Zombinmatronic version of Sanity Cat from HvM actually has a calico stuffed into it; this supports the theory that someone tampered with the animatronics to make them hostile to anyone.
-Ironically, in some games, Sanity is a hallucination, but in an upcoming game, Sanity reduces hallucinations.