Repel Gel

From the Super Mario Wiki

Repel Gel is an item found in Paper Mario. The gel makes Mario intangible for three turns. Repel Gel is very rare and can be found in only a few select areas and from ? Mark Li'l Oinks.

In Paper Mario: The Thousand-Year Door, Repel Gel was replaced by Boo's Sheet, which is a much more common item.