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. The 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 the Polterpup ends up stealing important keys.

Luigi's Mansion: Dark Moon
The 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, the Polterpup winds up getting the Amethyst stuck in a tree, after which he leaves the Manor.

Later, the Polterpup appears in Haunted Towers during the mission Pool Party. After Luigi finds the special key in the pool, the Polterpup runs up and playfully takes away the key. However, unlike his fellow ghosts, the absence of the Dark Moon's pacifying effects renders the 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, the Polterpup will even lick him back to health as long as Luigi has a Gold Bone.

After the 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, the Polterpup manages to escape from the Poltergust 5000 and flees yet again. It is at this point that Professor E. Gadd surmises that the Polterpup is lonely and is looking for someone to play with, even if his attempts to do so unintentionally cause trouble for Luigi.

The Polterpup returns in the Old Clockworks during the mission Play Catch, where he steals the second Clock Hand from a group of Greenies and swallows it whole. Luigi has to chase after him yet again before finally catching him in the Poltergust 5000. Despite this, the Polterpup manages to escape yet again before he can be stored away. The Polterpup returns yet again in the Treacherous Mansion during the mission Front-Door Key. Just before Luigi can go through the front door, the Polterpup jumps out of the keyhole and knocks the key out of Luigi's hand. He flees to the mansion's basement, forcing Luigi to track him down again. This time, the 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, the 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 the Polterpup up, much to his delight. After the 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, the Polterpup appears in the Nintendo 3DS remake. When Luigi's amiibo is scanned, the Polterpup appears when Luigi has lost all of his health by eating the Gold Bone to recover Luigi.

Luigi's Mansion 3
The 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, the 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 flooded a room in the Boilerworks and Luigi can hardly swim with the Poltergust on his back, the Polterpup pulled him out of the water. Later, the Polterpup searches together with Luigi for the Polterkitty after she swallows 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, the Polterpup unintentionally saves Luigi from King Boo by pushing him aside before he is caught in a painting with Mario and Princess Peach (He was chasing a bat when he jumped onto Luigi). When the hotel collapses, the 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.

At the end of Luigi's Mansion: Dark Moon, the 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, the Polterpup no longer has pupils. In the FAQ section of the guide in Luigi's Mansion 3, Professor E. Gadd states that he is researching what breed of dog the Polterpup is, and guesses that he is "...something like a silver retriever!"

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

Personality
The Polterpup's typical acts like a normal dog, and is playful, loyal, and energetic. Unlike the other ghosts in Luigi's Mansion: Dark Moon, Polterpup is friendly, harmless, and acts more like a nuisance than a threat to Luigi during his quest to retrieve the pieces of the Dark Moon. By the end of the game, the Polterpup is remorseful of his actions, and is quick to recover from his sadness, especially after befriending Luigi. E. Gadd speculates the Polterpup is lonely, and his interactions with Luigi could have been a desire to gain the attention he craves. After Luigi adopts him, he develops a strong sense of loyalty towards Luigi.

This loyalty is expanded upon in Luigi's Mansion 3 where the Polterpup acts like a guide to Luigi in the beginning of the game, and occasionally coming to Luigi's rescue when he is in true danger, including saving him from drowning and falling to his death. The Polterpup's mischievous and hyperactive behavior still is intact as shown when he eats Mario's cake and appears randomly, unintentionally, spooking his owner. Other than his master, the Polterpup is extremely friendly and loving towards those around Luigi, perking up and smiling around them, and was willing to befriend Polterkitty, only to be rejected by her, which saddened him.

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

 * According to a Swapnote message sent by Shigeru Miyamoto, the Polterpup is his favorite ghost.