Invisible

Invisible is a status effect in the Paper Mario series. When affected, the character cannot be attacked; all attacks will go right through.

Paper Mario
Repel Gels are the main way to become invisible in this game. The status is indicated by a square divided horizontally into black and white segments. A circle is also divided in half into black and white, with the white half on the black background and the black half on the white background. The entire symbol switches shades repeatedly.

Paper Mario: The Thousand-Year Door
In this game, Boo's Sheet replaces the Repel Gel. The indicator in this game is a semitransparent smiling face.

In addition, some Recipes such as the Kooky Cookie and Love Pudding can cause the eater to become invisible.

Super Mario 64
In this game, using the Vanish Cap, Mario will turn invisible. This allows him going through certain walls, and not getting harmed. However, it only lasts for a while.