Cat Mario or the Cat Bell is a new power-up found in Super Mario 3D World. It can be used by all the playable characters in the game: Mario, Luigi, Princess Peach, Toad, Rosalina, and even Bowser (in the final boss). Cat Peach appears as a Mario Kart 8 DLC character.
The main use of the Cat power-up is the ability to climb up walls and even the Goal Pole at the end of the level. It can also pounce on enemies, slide into them, and scratch them, making it the only actual melee attack in the game.
The Lucky Bell power-up allows the character to become Lucky Cat Mario (Luigi, Peach, etc.). This item can let the player turn into a golden statue similar to the Tanooki Statue from Super Mario 3D Land.
The final boss of the game, Meowser, is Bowser under the influence of a Cat Bell. This allows him to climb up walls and with the help of the other new item in the game, the Double Cherry, Meowser can clone himself.