Gooigi

Gooigi is a clone of Luigi who appears in the Nintendo 3DS remake of Luigi's Mansion and 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 or Poltergust G-00. He also has a letter G on his hat instead of an L. He was created by Professor E. Gadd during Luigi's Mansion: Dark Moon and sent back in time. In addition to Gooigi's voice being a warbled, higher-pitched version of Luigi's, he is apparently edible and may taste like coffee.

Background
During Luigi's Mansion: Dark Moon, Professor E. Gadd was experimenting on a strange new blue liquid he gathered from other ghosts, as he relates in his research journal. Believing this liquid to be a key to many new inventions, he performed tests on the liquid, but it did not react to anything: heat, cold, being poked with iron, being poured on rubber, or E. Gadd's morning donut. When a "stealthy ghost" sneaked up behind him, E. Gadd spilled his Lunoman Greenie blend coffee into the liquid, causing 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.

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 behind bars, past spikes, and through grates. However, he melts if he touches water and disintegrates if he touches fire. Gooigi is unable to open doors, although he can still insert keys in them to unlock them. Gooigi can also vibrate when Luigi is near a door and enters a room where a Boo is hidden; the closer Luigi is to the Boo, the more Gooigi vibrates, similarly to the Game Boy Horror's Boo Radar in Luigi's Mansion. He also has only 25 HP; however, whenever Gooigi takes damage, he momentarily recovers his HP at a steady pace. 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. He also appears in ScreamPark, where the player can choose him or Luigi as a playable character. According to E. Gadd when supplying Luigi with Gooigi, he created him to have a helper in catching ghosts on his spare time, since E. Gadd's advanced age prevented him from being as capable of doing ghost-hunting himself as before.

At the end of the game, Gooigi contributes in rebuilding the destroyed hotel, and he stays behind with E. Gadd and the ghosts while everyone else leaves.

Super Smash Bros. Ultimate
While never seen in 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 (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."

Luigi's Mansion 3

 * European website bio: "This gooey, green doppelganger can use his...err...gooiness to squeeze through tight spaces, walk across dangerous spike-laden floors, help lift heavy objects and overcome all sorts of contraptions."
 * In-game bio: "A doppelganger of Luigi that is stored inside the Poltergust. Made from a substance created from ghost energy, called "goo," Gooigi looks almost exactly like Luigi!"

Play Nintendo

 * Luigi's Mansion 3 Trivia Quiz: "Professor E. Gadd discovered the green element known as Goo, then used Luigi’s biological data to create an exceedingly green copy of Luigi."

Trivia

 * Before Gooigi's creation, E. Gadd listed "Goo" as a new element on the . However, due to how Gooigi was created, it would by definition not be an element but a compound or mixture.
 * To further this idea, all elements after (element 83) are radioactive. According to E. Gadd, Goo is stable. However, Goo is element number "00" (a reference to its name when combined with its symbol: G 00, which is also used in "Poltergust G-00").
 * 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.
 * Concept art of Luigi's Mansion 3 shows Gooigi with an L on his cap instead of a G. It is also the only depiction of Gooigi in which he shows any kind of emotion, namely a fearful look on his face.