The Brain is a ghost in the Luigi's Mansion series, originally appearing in Luigi's Mansion: Dark Moon, and in a fan game, Luigi's Mansion 3: Polter Pursuit as one of the main antagonists.
The Brain appeared in Luigi's Mansion: Dark Moon as the final boss of the ScareScraper mode. He is an oversized Poltergeist/Boffin. He is always fought on the roof. Unlike other Poltergeists, he's able to produce a large forcefield. He also has 300, 500, 700 or even 900 HP.
The Brain appeared in Luigi's Mansion 3: Polter Pursuit as the main antagonist. He worked together with King Boo to take over Evershade Valley and Boo Woods, and beyond. He is first teased at in the Treacherous Mansion, at the end of it's fifth mission. Aftet defeating King Boo, E. Gadd discovers The Brain yet has to be defeated. He resides in the ScareScraper, and Possessor aura blocks the path to it. After doing several mansions, Luigi achieves the ScareScraper's front door key from the Possessors, also destroying the Possessor aura. As soon as he attempts to open the front door, though, Slammers take the key to the tower's huge basement. Upon reaching the top floor, The Brain hides in a chest. Upon opening it, The Brain pops out and fights Luigi. He fights the same as in Dark Moon, but he has 9753 HP, and the forciefield does inflict damage. Upon defeating him, The Brain teleports Luigi to his own dimension. After a rematch with a fake King Boo, The Brain fights Luigi, this time with a green suit and a red brain, making him more powerful. To defeat him, the player must avoid the bombs he throws, then suck them up and blow them to the Poltergeist, stunning him (the Strobulb doesn't work on him). The Brain will first use a Paranormal Shield (∞ HP). After it's destroyed, The Brain will again have ∞ HP. Once a Power Surge is used, the green skin is pulled off, and The Brain's true 900 HP is revealed. The Brain has several other attacks (TBA).