Gooigi

"Allow me to introduce this green doppelgänger. I call him Gooigi!"

- Professor E. Gadd

Gooigi is a clone of Luigi who appears in the Nintendo 3DS remake of Luigi's Mansion and again in Luigi's Mansion 3. He appears in Luigi's image, but is entirely composed of green goo, hence his name, and wields a gooey version of the Poltergust 3000 and Poltergust G-00. He also has a letter "G" on his hat instead of an "L". He was created by the future Professor E. Gadd from Luigi's Mansion: Dark Moon. He is apparently edible, and may taste like coffee.

Background
During Luigi's Mansion: Dark Moon, Professor E. Gadd was experimenting on a new, strange blue liquid he gathered from other ghosts, as he relates in his research journal. Believing this liquid is a key to many new inventions, he performed tests on the liquid, but it did not react to anything: heat, cold, poking it with iron, pouring it on rubber, and E. Gadd's morning donut. When a "stealthy ghost" snuck up behind him, E. Gadd spilled his Lunoman Greenie blend coffee into the liquid, which caused it to turn green and stabilize into a goop form, which E. Gadd calls "Goo."

After further tests, the goop took the form of Luigi and was named "Gooigi" and sent back in time to the events of the original Luigi's Mansion, outsourcing E. Gadd's testing to his and Luigi's past selves. Three collected Dark Moon pieces are shown on the monitor behind the future E. Gadd, indicating that this occurred after Luigi had cleared out the Haunted Towers and begun exploring the Old Clockworks.

Luigi's Mansion (Nintendo 3DS)
In Luigi's Mansion, Gooigi serves as the second player in the game's new multiplayer mode. He begins with only 50 HP and is vulnerable to fire, burning and melting very easily, but can revive indefinitely if his health reaches zero. When Gooigi is not being played by a second player, he is seen in the background of The Lab staring at some test tubes.

After the end credits, the future E. Gadd calls a second time to look at Gooigi's stats if the game was played in multiplayer.

Super Smash Bros. Ultimate
While never seen in his humanoid form, Gooigi is technically present in Super Smash Bros Ultimate as he is stored inside Luigi's Poltergust G-00, which he uses for his grabs and Final Smash.

Luigi's Mansion 3
Gooigi reappears in Luigi's Mansion 3, where the player can use him to reach certain areas that Luigi cannot reach, such as areas past spikes. However, he melts if he touches water and disintegrates if he touches fire. He also has only 25 HP. In co-op mode, Gooigi is controlled by the second player. He also appears in the Scarescraper, where multiple colored Gooigis can appear for multiplayer; Gooigis come in blue, gold, and pink colors.

Luigi's Mansion (Nintendo 3DS)

 * Digital manual description: "Gooigi cannot open doors. He has half the health of Luigi, but if he is defeated in battle, he can revive after a short time."

Trivia

 * Before Gooigi's creation, E. Gadd lists "Goo" as a new element on a periodic table. However, due to how Gooigi was created, it would by definition not be an element, but a compound or mixture.
 * In the last image of "XXX-003 The Birth of Gooigi", Gooigi is shown to be having a much clearer "G" emblem on his cap, as opposed to the very faint "G" on his cap in-game.
 * The file name of an image in the official website for the Nintendo 3DS version of Luigi's Mansion misspells his name as "Guigi".