Cat's Room

The Cat's Room is a pavillion located inside the Sohma Estate, an isolation room where the person hosting the spirit of the Cat is locked up through their whole life. It is a part of the Curse that God had put on all of the animals of the zodiac.

Use
The purpose of the Cat's Room is to lock up the host of the spirit of the Cat as punishment for disobeying God and going against them. The Cat had originally seen God's plan as a failure, predicting it to ultimately not work, and that the other animals would turn on God for forcing all of them to be with him/her.

God put a curse on the Cat, thinking they were a traitor to everyone. With each rebirth of the Cat's spirit, the person inhabited by them will be forced to be trapped in this room from some point in their life onwards. When they are locked up, the other animals are forced to join God in an everlasting banquet until they all die.

The room, while never truly seen, serves as a place of isolation for the Cat, making sure their life is awful for trying to keep the rest of the Zodiac from forming a bond with God.

History
It is known that every person cursed by the Cat spirit has been confined into the Cat's Room at some point. i.e., Kazuma Sohma's grandfather was isolated in the Cat's room most of his own life up until his death, though it's never really stated when it began. He once offered his grandson Kazuma a candy, but Kazuma harshly rejected it, saying it is cursed; the gentleman didn't hold it against him, however.

Years later, the now adult Kazuma has never forgiven himself for his cruel treatment of his grandfather and has become very protective of the actual Cat, his younger relative and adoptive son Kyo Sohma. Kyo's own lock-up has been delayed thanks to Kazuma's personal intervention, but there's a limit to such delay: Kyo's high school graduation.

Story Overview
Kyo, knowing how his life will play out, lashes out at Yuki Sohma alias the Rat of the Zodiac, hating on him for being the closest to God, and for being the one who tricks him in the old story. Because of the other part of the curse, the True Form of the Cat spirit, Kyo is seen wearing a Juzu Bead made out of human bones and blood.

Before the series begins, Kyo makes a bet with Akito: If Kyo is able to beat Yuki, even just once, in anything before they graduate, the Cat can be included in the Zodiac. However, if Kyo fails to complete his end, he will be locked in the Cat's Room until he dies like all the Cats before him.

Later in the story, Isuzu "Rin" Sohma (the Horse of the Zodiac) is temporarily imprisoned inside the Cat's Room. This is because, under Ren Sohma's influence, she attempted to steal a lacquered box that is precious to Akito under the promise that Ren would tell her how to break the curse itself. When Akito walked on Rin doing so, she was so enraged that she grabbed Rin by her long hair and threatened to blind Hatsuharu, but Rin said she'd rather be locked up and away instead.

As the days and weeks pass, the already troubled Rin is so emotionally unstable over her confinement that she pretty much stops eating or reacting to anything. She's locked up for around three or four months, while everyone else thinks Rin has been hospitalised again due to her terrible health.

Eventually Kureno Sohma realises that one of the maids is secretly bringing food to the Room itself (no one is supposed to approach it if it's empty) and confronts her: she tells him that she and her father are taking care of Rin under threat of being fired if they don't keep the secret, adds that Rin has stopped eating and she fears for her life, and gives him the key to the room while begging him to save her. He enters the Cat's Room and finds the malnourished and despondent Rin there, deciding to have her taken to the hospital behind Akito's back with Hatori's help.

Akito later destroys the Cat's Room, and thus, Kyo is never trapped in it. This comes at a surprise to him. Soon after, the curse on all of the animals is lifted, including the cat, meaning the room won't be rebuilt.

Trivia

 * Akito talked with Kyo's father before she decided to destroy the cat's confinement room.
 * On the outside, the Cat's Room seems to be merely a warehouse or a pavillion in a corner of the Sohma estate. On the inside, however, it's more akin to an old Japanese prison room, even having wooden bars.