Boo Mario

Boo Mario is a form of Mario that debuted in Super Mario Galaxy. As its name implies, the Boo Mario form is Mario disguised as a Boo, although his hat and face remain respective of his normal form. Boo Luigi, the equivalent form of Luigi, shares the same traits though with his respective features and a more oval-shaped body. In Super Mario Galaxy, Luigi has a worried look on his face, referencing his fear of ghosts, though he does not in Puzzle & Dragons: Super Mario Bros. Edition.

Super Mario Galaxy
In Super Mario Galaxy, in order to become Boo Mario, Mario must grab a Boo Mushroom. With this transformation, Mario can turn himself invisible and pass through certain walls and gates by shaking the Wii Remote, levitate by pressing the button, and descend by holding. The walls Mario can pass through as Boo Mario are similar to the ones he can pass through with Vanish Cap in Super Mario 64. Mario can also understand the language of the Boos, called Booish, meaning signs written in that language can be understood by him once he transforms. His Boo form also attracts other Boos, causing them to follow him around and allow him to make use of them for various purposes. The transformation is lost if Mario takes damage, passes through light, touches water, or touches a Launch Star.

Mario first uses this transformation in the Ghostly Galaxy to rescue Luigi. Because Luigi is afraid of ghosts, he cowers when Mario approaches him in Boo form; therefore, Mario must return to his normal form beforehand. It is also used in Boo's Boneyard Galaxy and Sand Spiral Galaxy.

Super Mario Galaxy 2
This power-up returns in Super Mario Galaxy 2, in the mission "Haunting the Howling Tower" of Boo Moon Galaxy, where the power-up is very widely used.

Super Smash Bros. for Wii U
Boo Mario appears in Super Smash Bros. for Wii U as a trophy. As with most trophies in the game, it can be obtained when it appears at random in one of the game's modes, such as Vs., Trophy Shop, or others.

Super Smash Bros. Ultimate
Boo Mario makes an appearance in Super Smash Bros. Ultimate as a Novice-class support spirit, increasing the fighter's jump height and move speed but making them easier to launch when equipped. In the spirit's battle, the player faces an invisible Mario on the Dracula's Castle stage.

Boo Mario's core can be combined with Ezlo's core to summon Cappy's spirit, or with a Shy Guy's core to summon Roger the Potted Ghost's spirit.

Puzzle & Dragons: Super Mario Bros. Edition
Boo Mario, as well as Luigi's equivalent form, also appears in Puzzle & Dragons: Super Mario Bros. Edition. He is unlocked by clearing.

Super Mario Galaxy

 * Trading card bio: "In the deep recesses of the Ghostly Galaxy, strange and frightening spirits are after Mario. To help Mario navigate this scary place, he can grab a Boo Mushroom and power up to become Boo Mario! This allows him to become invisible, float like a Boo, and pass through walls."