Boo Mario

Boo Mario is a form of Mario that appears in the game 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. 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 and levitate by pressing the button. Mario can attract Boos as well (if he approaches one, it will seemingly fall in love with him and chase him), and use them for various purposes. He can also understand the language of the Boos, called Booish, meaning bill boards written in that language can be understood by him as well. The transformation is lost if Mario takes damage, passes through light or touches water. 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. This power-up does appear in Super Mario Galaxy 2, but in only one galaxy: the Boo Moon Galaxy in the mission "Haunting the Howling Tower", where the power-up is very widely used.

Super Mario Galaxy Trading Card Description
''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.''

Trivia

 * In Super Mario 64 DS in Big Boo's Haunt, if the player uses a character other than Luigi, there is a Boo that wears Luigi's Cap (which could be obtained after defeating the Boo), much like Boo Luigi.
 * Mario (or Luigi) can lose the Boo form when touching a Launch Star, but it's not mentioned in the game's instruction booklet.
 * Humorously, Luigi's Boo form appears tired and lethargic. This is most likely due to his constant fear of ghosts.
 * 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.