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Gooigi is a clone of Luigi who appears in the Nintendo 3DS remake of Luigi's Mansion. 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. He was created by a future Professor E. Gadd.
In the game, 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.
During Luigi's Mansion: Dark Moon, Professor E. Gadd was experimenting on a new, strange, blue liquid he gathered from other ghosts. Believing this liquid is a key to many new inventions, he performed tests on the liquid, but it didn't 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 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". While Luigi was exploring the Old Clockworks, it was 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.