You can play (almost) the entire Luigi’s Mansion 3 campaign co-op but first you need to unlock Gooigi. He’s unlocked at the end of your first official set of objectives. This means after the Intro Walkthrough and after you retrieve E.Gadd’s briefcase and bring it back to his lab.
After a brief tutorial on how to use Gooigi, you’ll be ready to play co-op.
To play co-op hit the plus button > select “Co-Op” > select co-op (Again) > click in so that both controllers (or joycon) are detected and hit A. Just like that you’re playing co-op!
To switch back to single-player simply press the plus button > select solo and repeat the steps.
Janet Garcia is an editor and strategy guide writer at IGN. She has played both Luigi’s Mansion, Luigi’s Mansion Dark Moon, and now Luigi’s Mansion 3. After speaking to Nintendo representatives about Luigi’s Mansion 3 at E3 2019 and doing extensive research on Gooigi, she felt more than ready to take on every floor of this hotel. For more haunted content, you can follow her on Twitter @Gameonysus.