Gooigi is a gooey-textured version of Luigi's model. He wears a dark green shirt, evergreen overalls, yellowish green shoes and spring green gloves. His hair is more of an lime green.
Professor E. Gadd's Journal
An Accidental Discovery & Gooigi's Birth
E. Gadd begins by making a short introduction on a "strange liquid energy" collected from ghosts he had befriended. He posits that said energy is "the key to many, many new inventions," but admits that he initially struggled to uncover its uses. His failed experiments with the energy included poking it with iron, pouring it on rubber, and even absent-mindedly dipping a donut into it.
The professor recounts that later during his research, he was startled by a Hider ghost that had escaped from its confinement, causing him to spill the coffee he was drinking into a vial containing ghost energy. He claims that the coffee, described as a "Lunoman Greenie blend from Evershade Valley," quickly reacted with the energy inside the vial, leading to the creation of a green viscous substance. E. Gadd recognized the substance as a new chemical element and called it "Goo," which he then assigned the "00" entry in the periodic table.
E. Gadd writes that he could teach Goo to collect ghosts with his invention, the Poltergust 3000. He first created a mold that he could pour Goo into a humanoid form. When he removed the mold, Goo was in a human shape, but returned to its original form moments later. The professor then used the Pixelator that contains Luigi’s biological data and converted it into an electrical signal. He then was able to input the signal into the Goo. The result was that the Goo has shape-shifted into a green copy of Luigi.
E. Gadd then showed the Goo some footage of Luigi catching ghosts. The Goo began to imitate Luigi’s movements, and even grew a perfectly functional replication of the Poltergust out of its body. E. Gadd decided to call it "Gooigi" and felt determined to find the perfect place to test "his" ghost-hunting abilities.
The Nostalgic Mansion
E. Gadd admits that he had difficulty in thinking about an optimal place where Gooigi could be tested out. He also had to come up with a plan to obtain more ghost energy from Evershade Valley in order to confer Gooigi paranormal capabilities. While pondering over a solution, he realized after a sip of his Lunoman Greenie coffee that he could send Gooigi to the mansion where he originally worked with Luigi. However, since the mansion had vanished long before, E. Gadd had to program his Pixelator so that it could connect to the past and make revisiting the mansion possible. In this way, he contacted himself and Luigi from the past, asking them for help, and outsourced Gooigi to them afterwards.
When he contacted Gooigi after a while for a check up, he discovered that he was very successful in his tests and had collected a large amount of ghosts. As a final note, E. Gadd writes that he is thinking about other possible interactions between ghost energy and Goo.
A New Power
Professor E. Gadd writes that he began his research in improving Gooigi's qualities "beyond that of a normal person" with the large ghost supply obtained by Gooigi from the original Luigi's Mansion. As he analyzed the ghost energy, he discovered that ghosts from the original mansion were structured differently from those in Evershade Valley and became clear to him that there are various types of ghost energy based on their molecular structure. E. Gadd remembers that he still needed a way to benefit Gooigi from the new discovery, and since Goo is different enough from ghost energy, he would not be able to give him ghostly properties like vanishing or going through solid walls.
E. Gadd knew that Gooigi's viscosity could be significantly enhanced to enable him to slip through tight spaces, and even considers this as "one of [his] greatest inventions to date;" however, he needed the optimal mix of ghost energy applied to Gooigi. He then began testing various mixtures of ghost energy and Goo. Most of the experiments either had no effect on Gooigi or made him more rigid. After many more tests, the final formula let Gooigi retain his form while still giving him a fluid quality, allowing him to pass between bars. Satisfied by the success of this experiment, E. Gadd decided to take a break with a cup of Lunoman Greenie coffee blend.
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: Dark Moon
In Luigi's Mansion, Gooigi serves as the second player in the game's new multiplayer mode. 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 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.
E. Gadd began his research into addressing Gooigi's vulnerability to fire or water. After reducing the density of the Goo to no avail, he then attempted to increase the density instead. However, he was startled by Polterpup, which causing him to spill the compound onto his head. The compound began to harden and cause E. Gadd to suffocate, but he was quickly able to run to the shower and wash it off with water.
After performing many tests, E. Gadd eventually realized that eliminating Gooigi's weaknesses was a task beyond even his own expertise, but his experiments had at least subtly improved Gooigi's viscosity. E. Gadd summed up the results of his tests:
While Gooigi can still dissolve by being directly burned or soaked, he can withstand steam and vapor.
Gooigi can withstand light physical attacks, but cannot endure powerful blows. Gooigi is capable of applying subtle forces to objects, such as opening drawers and treasure chests, but cannot perform actions such as turning doorknobs. Still traumatized by his near-death experience, E. Gadd sipped a cup of Lunoman Greenie coffee as he pondered his next experiment.