Boo Mario

Boo Mario is a form Mario takes on in Super Mario Galaxy via touching a Boo Mushroom. It can only be found in the Ghostly Galaxy and Sand Spiral Galaxy. Mario must find it to save Luigi in a ghost mansion. If the player shakes the Wii Remote, Mario will become transparent and walk trough some walls. He can also levitate a little higher. If Mario runs into a light or gets hit by another Boo, he will revert to normal. Mario also races the Spooky Speedster throughout the galaxy.

Super Smash Bros. series

In this series, it is only found as a trophy.

Super Smash Bros. for Wii U

Are you afraid of ghosts? Maybe it will help to see the world as they do. Become Boo Mario with a Boo Mushroom, and then explore their world! Shake your Wii Remote to make Boo Mario go through walls. Boo Mario blends in OK, but even as a ghost he keeps his trademark hat. The other Boos don't seem to mind, thankfully.
Wii - Super Mario Galaxy - 11/2007
Wii - Super Mario Galaxy 2 - 05/2010