The Wandering Spirit is a key item that appears in Wario: Master of Disguise.
The Wandering Spirit is a spirit that is green in color and resembles a flame.
The Wandering Spirit can be found in Blowhole Castle inside of a purple treasure chest in a room with Stuffy the 5th is located. When Wario ran into a wall of spirits, Tiaramisu informs him that the spirits are missing their friend. The Wandering Spirit is indeed that friend and reuniting it with the other spirits cause them to be at ease and allowing Wario to finally face Stuffy the 64th.