Gooigi is an invention of Professor E. Gadd in the Luigi's Mansion series. It is a doppelgänger of Luigi made of ecotoplasmic goo and controlled by his life force. In single player, when Gooigi is created Luigi hunches over and becomes limp and the player controls Gooigi, implying that Luigi's soul has fully transferred into the goo. Gooigi can assist Luigi in tasks and is able reach places that Luigi cannot himself, such as over spikes and through grates. In Luigi's Mansion 3, Gooigi can also be controlled by another player as a co-op partner.
Gooigi first appears in the 3DS remaster of the original Luigi's Mansion although the original idea for the concept was created as part of development for Luigi's Mansion 3, according to the latter's game director Yoshihito Ikebata.