Luigi's Mansion 3 will be out later this year, and on today's E3 2019 Nintendo Direct they showed more of the game including Gooigi, an abomination against god and science.
It appears that there is a co-op mode in Luigi's Mansion 3, and Gooigi is the key to you and a friend going on adventures together throughout the haunted hotel Luigi finds himself in this time.
As a gooier, less human version of Luigi, Gooigi can pass through bars and spikes however is weak to water, which makes sense because water can be purified into a holy defense against this servant of satan. There is no way this slimey homunculus is anything less than malevolent. Even if it is controlled by your friend.
The rest of the game is full of improvements over the first two, as Luigi can now jump using the Poltergust with a Burst function. No, sorry, sorry we're still not over Gooigi. That is some seriously, seriously cursed stuff.
Check out the Luigi's Mansion 3 trailer below once it's available, but please make sure you have some purified holy water handy when you do.