You have to beat Joey's Castle's 2nd story campaign on hard mode (which unlocks after you see the credits for the first time), but the unlock flag is bugged in the overseas version of the game. To get Dark Luigi to unlock properly, you'll need to get the Japanese v1.1 revision of Joey's Castle (ideally with an English translation patch) and beat the hard mode using that version; once unlocked (you'll know you unlocked him if the game shows you a CG render of him after the credits), Dark Luigi becomes playable for new game plus (if you're playing without the translation patch or you're using an early revision of the patch, he may be listed as 「ダークルイージ」, romanized as "Dākuruīji").

On real hardware you can glitch the overseas version of Joey's Castle into triggering the unlock by tilting the console's backup save RAM cartridge as you draw a Topp's card; doing so can trigger a buffer overflow and forcibly activate all of the unlock flags for your save file, although there's also a small risk of corrupting all saves on your save cartridge, so be careful.

Dark Luigi is super fun to use, but most locals will probably ban him since he's too unbalanced.

Don't listen to that other guy. They patched Dark Luigi out in v1.1.

Originally posted by Hotdawg:

Don't listen to that other guy. They patched Dark Luigi out in v1.1.

Yeahg, and it's a good thing too. I've heard Dark Luigi was a character made to test the various features of the game. I mean, infinite knives and AKs, 999 health, the ability to instantly enter and exit hell whenever the player felt like it, the character transformation persisting through deaths, and the ability to draw ANY card for ANY reason? Of course they'd want to patch him out before the game was released to the public! Dude was about as well balanced as the entirety of world of warcraft.

That being said, I'm hopeful they add Dark Luigi back in a later update as a properly balanced character that's fun to play.

You gotta type in the following as a cheat code "stun seed sugnoma" then spin around the Golden Fren 3 times with the C-Stick, and then you look up where he will appear. Beat a round of Joey's Castle and Dark Luigi is unlocked.

Dark Luigi never existed. He's just some mandela effect creepypasta analog horror character that was made by some... wait somebody's at my door, hang on...

why yes, this is the residence o-OH GOD ITS DARK LUIGI AND HE'S COMING TO EAT ME!
HELP HELP HELP HELP HELP!! I NEED WATER!! AGGA!!!!!!!!!!!!! *dies*