Princess Lipid

Princess Lipid is a strange white figure in Mario & Luigi: Bowser's Inside Story that saves Luigi from falling deep into mucus. She will pull Luigi and a treasure chest out of the mucus and ask Mario which he dropped. However, before he answers, Princess Lipid make the conclusion that Mario dropped the treasure chest. Mario quickly says "No", and Princess Lipid ends up giving both Luigi and the chest to Mario.

Trivia

 * Though Princess Lipid has the term princess in front of her name, she is never revealed to be royalty.
 * Due to her Japanese name, she may be made out of mucus.
 * Her name is taken from the word "lipid", which are a group of molecules such as fats and waxes.