Polterpup

Polterpup (known as ??? in E. Gadd's Vault) is a specific Polterpup who appears throughout Luigi's Mansion: Dark Moon and Luigi's Mansion 3. Unlike normal Polterpups, he has a red collar instead of blue.

Luigi's Mansion: Dark Moon
Polterpup first appears in Gloomy Manor, where he is playing in the Mudroom Exterior and discovers an Amethyst after digging around the ground. While briefly playing with it, Polterpup winds up getting the Amethyst stuck in a tree, after which he leaves the Manor.

Later, Polterpup appears in Haunted Towers during the mission Pool Party. After Luigi finds the special key in the pool, Polterpup runs up and playfully takes away the key. However, unlike his fellow ghosts, the absence of the Dark Moon's pacifying effects renders Polterpup nonviolent and mischievous, rather than hostile and aggressive. This is evident when Luigi briefly plays with him and, in the event Luigi loses all of his Heart Points, Polterpup will even lick him back to health as long as Luigi has a Gold Bone.

After Polterpup steals the special key, Luigi has no choice but to chase after him in the mission Doggone Key to retrieve the key. Luigi finally catches him, but before he can store him in the Ghost Container, Polterpup manages to escape from the Poltergust 5000 and flees yet again. It is at this point that Professor E. Gadd surmises that Polterpup is lonely and is looking for someone to play with, even if his attempts to do so unintentionally cause trouble for Luigi.

Polterpup returns in the Old Clockworks during the mission Play Catch, where he steals the second Clock Hand from a group of Greenies. Luigi has to chase after him yet again before finally catching him in the Poltergust 5000. Despite this, Polterpup manages to escape yet again before he can be stored away. Polterpup returns yet again in the Treacherous Mansion during the mission Front-Door Key. Just before Luigi can go through the front door, Polterpup jumps out of the keyhole and knocks the key out of Luigi's hand. He flees to the basement, causing a long and paranormal chase through the odd rooms. This time, Polterpup is able to be properly kept within the Ghost Container thanks to E. Gadd's repairs.

Following King Boo's defeat and the restoration of the Dark Moon, Polterpup and his fellow ghosts are finally released from the Ghost Container. Upon being released, he unhappily approaches Luigi in acknowledgement of having caused his fair share of trouble during the absence of the Dark Moon. However, Luigi nevertheless cheers him up, much to Polterpup's delight. After Polterpup takes a celebratory photograph alongside Luigi, Mario, E. Gadd, the Toad Assistants and a few of his fellow ghosts, he is adopted by Luigi and accompanies him back to his home as his new pet.

Luigi's Mansion (Nintendo 3DS)
Despite not appearing in the original Luigi's Mansion, Polterpup appears in the Nintendo 3DS remake. When Luigi's amiibo is scanned, Polterpup appears when Luigi has lost all of his health by eating the Gold Bone to recover Luigi.

Luigi's Mansion 3
Polterpup returns in Luigi's Mansion 3, this time as a tutorial guide. He remains as Luigi's pet, and accompanies him and the others to The Last Resort hotel. After Luigi falls down the shaft, Polterpup wakes him up by licking his face. He then shows Luigi where the new Poltergust G-00 is and teaches him how to use it. Later, when the ghost Clem filled the room with water, and Luigi can hardly swim with the Poltergust on his back, Polterpup pulled him out of the water. Later, Polterpup searches together with Luigi for the Polterkitty after she steals an elevator button for the second time, and will assist him in reaching two of the trapped Toads after the respective boss fights. At the end, Polterpup saves Luigi from King Boo by pushing him aside before he is caught in a painting with Mario and Princess Peach. When the hotel collapses, Polterpup saves Luigi for the second time by catching him before he crashes to the ground. In the end, he goes back with Luigi to the Mushroom Kingdom.

Super Smash Bros. series
Polterpup appears as a trophy in Super Smash Bros. for Nintendo 3DS, and as an Advanced-level, support spirit in Super Smash Bros. Ultimate.

Luigi's Mansion: Dark Moon

 * Ghost Container description: "I haven't come up with a name for this mischievous and adorable Polterpup yet. He's caused us a whole mess of trouble, but I think the playful tyke just wants someone to take him in."

Luigi's Mansion 3

 * In-game bio: "A ghost dog who loves Luigi and is always by his side. He'll come to Luigi's aid when he's in truly dire situations!"
 * North American website bio: "Polterpup helps track down important items for Luigi. This adorable, ghostly pup will also give Luigi hints on where to go."

Trivia

 * Polterpup is notably the only ghost that Luigi does not show any fear of in Luigi's Mansion: Dark Moon. Instead, Luigi will either be friendly toward him or view him as a mere nuisance, the latter of which is evident when he says, "Bad doggy!" whenever Polterpup ends up stealing important keys.
 * At the end of Luigi's Mansion: Dark Moon, Polterpup gains pupils like the other restored ghosts and retains them when he goes to live with Luigi. However, in Luigi's Mansion 3, despite still being friendly, Polterpup no longer has pupils. This is likely merely an oversight on the part of the designers, and not an intentional part.
 * In the FAQ section of the guide in Luigi's Mansion 3, Professor E. Gadd states that he is researching what breed of dog Polterpup is, and guesses that he is "...something like a silver retriever!"
 * According to a Swapnote message sent by Shigeru Miyamoto, the Polterpup is his favorite ghost.