Luigi

Luigi is a playable character exclusive to Super Mario 64 DS.

Apperance
Luigi is a tall skinny plumber, who is almost the exact opposite of his brother Mario, who is shorter and plumper. Luigi wears a green shirt and hat and blue overalls. He also sports brown shoes like Mario does. Luigi also has a more French/Italian, wavy styled mustache compared to Mario's bushy one. Luigi has a circular nose and blue eyes.

Super Mario 64 Rumors
In 2000, a rumor spread up on SM128C.com. It stated, that the statue in the fountain had a secret message reading "L Is Real 2401" or "Luigi is Real Feburary 4th 2001". Altough the rumor had been spread earlier, the site was the first major source to mention it. Actually, Feburary 4th, 2001 is the date the last Nintendo 64 game, Paper Mario, came out. In this game, Luigi is existant.

Despite the rumors, no matter what players do, without hacks, having Luigi in Super Mario 64 and thus playing as him is impossible.

However, in 2020, Luigi has been confirmed to appear in the game, but he still isn't playable outside of advanced hacking.

Role in Super Mario 64 DS
In Super Mario 64 DS, Luigi is one of the characters trapped by Bowser's minions, along with Mario and Wario. Once Mario obtains the key to Luigi's Room, Luigi will be playable throughout the remainder of the game.

How to Unlock Luigi
Note: Only Mario can be used to unlock Luigi.

Step 1.
Mario must make his way towards Big Boo's Haunt. (he's unscrapped in sm64 ds)

Step 2.
Mario must then climb his way to the attic of the level, and jump in the portrait of Luigi.

Step 3.
Mario will land in a room identical to the Underground Ride. Mario must listen to where King Boo's laughs are coming from. The correct order is Right Door, Left Door, Right Door, Left Door.

Step 4.
After making his way through the maze, Mario will be in a room with a long way of platforms and two elevators at the end. Mario must make his way to the elevator and go though the hole at the top.

Note: If Mario has the invisibility cap, he can jump through the Scared Boo painting and collect a Power Star.

Step 5.
Mario must head up to the mirror. King Boo will appear, and Mario must hit him three times to win. To defeat him, Mario must follow King Boo's shadow when he is invisible. Mario must the either punch or ground-pound on King Boo to injure him. Once he is hit three times, the Boo will give up and give you Luigi's Key.

Final Step.
Mario must make his way to the room where Yoshi unlocked him. Mario must then go over to the door with a green "L" over it. Once unlocked, Luigi comes out and is now playable.

Abilities
Luigi has many different uses than other characters.

Floating: If Luigi jumps and the player holds the jump button, Luigi will slowly float down to the ground.

No Wall Jump: Just like everyone else except for Mario, Luigi cannot Wall Jump.

Trivia

 * Luigi first appeared in the 1983 arcade game Mario Bros.
 * In the early days of the Mario series, Luigi was a recolor of Mario. This is especially noticeable in Super Mario Bros. and Super Mario Bros. 3.
 * Luigi has almost always been able to do higher jumps and fall slowly. This started, when Yume Kōjō: Doki Doki Panic was rebranded and released in the US and Europe as Super Mario Bros. 2. The sprites being replaced with Mario chracters, but their physics and abilties left intact, led to Luigi and Peach gaining their characteristic abilities for their own use.
 * Luigi has the same last name as Mario, being Mario. Shigeru Miyamoto confirmed this saying Mario and Luigi's last names are "Mario"
 * Luigi was celebrated extremely in 2013, also known as The Year of Luigi, which was Luigi's 30th anniversary.

Refrences
-the site where the rumors about L is real 2401 began.