Vanish Mario

From the Super Mario Wiki

Vanish Mario is one of Mario's special forms from Super Mario 64. Mario can transform into it by collecting a Vanish Cap, found inside ! Blocks, which are activated by triggering the ! Switch in the Vanish Cap Under the Moat secret area. As Vanish Mario, he is invisible to enemies (and transparent to the player), and is thus able to pass by them completely ignored and unharmed. As well as this, he becomes able to pass through cages, ice and certain walls. However, he can still be defeated by drowning or being crushed.

In Super Mario 64 DS, the Vanish form is granted to Luigi; however, the only difference is in Luigi's slightly different abilities and in that a Power Flower is required for the transformation, the ! Block being red.

Mario's Forms
Bee Mario | Boo Mario | Cape Mario | Dr. Mario | Fire Mario | Flying Mario | Frog Mario | Hammer Mario | Ice Mario | Invincible Mario | Mega Mario | Metal Mario | Mini Mario | Plane Mario | Rabbit Mario | Raccoon Mario | Rainbow Mario | Shell Mario | Small Mario | Spring Mario | Super Mario | Superball Mario | Superstar Mario | Tanooki Mario | Vanish Mario | Wing Mario