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.

Paper Mario: The Thousand-Year Door
In this game, Boo's Sheet replaces the Repel Gel.

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