Gooigi

Gooigi is a creature created by Professor E. Gadd in Luigi's likeness. He is composed of the element Goo, which E. Gadd discovered totally on purpose upon mixing liquid ghost energy with a Lunoman Greenie blend of coffee from Evershade Valley. He soon discovered that Goo seemed to be capable of intelligent response to outside stimuli, and he caused it to assume the form of Luigi by inputting his biological data into it in the form of an electrical signal. Gooigi soon developed ghost-catching abilities upon watching footage of Luigi's fearless exploits, transforming part of his own body into a replica of the Poltergust. After casually time travelling Gooigi to and from the past for testing, E. Gadd gave him the ability to move through grates and spikes, but also noted his weaknesses as follows:

"Weakness #1: Dissolves upon contact with water or fire. Note that direct contact with liquid H2O will cause this reaction, but rooms containing water vapor or steam do not.
Weakness #2: Loses consistency when struck by powerful attacks. While Gooigi is somewhat resistant to physical blows, he can not hold up to repeated or powerful attacks.
Weakness #3: Can't turn doorknobs. Things that can be opened with the minimum amount of force are fine, such as drawers or treasure chests, however."

Eventually, E. Gadd invented the Poltergust G-00 to allow for storage and deployment of Gooigi, as well as the ability to transfer the consciousness of the Poltergust's wearer to and from Gooigi's body. And such is the story of how Luigi got his own sidekick even greener than himself.

Also, Gooigi is edible and may taste like coffee.