Polterpup

Polterpup (known as ??? in E. Gadd's Vault) is a specific Polterpup who appears throughout Luigi's Mansion: Dark Moon. Unlike normal Polterpups, he has a red collar instead of blue. Professor E. Gadd surmises that Polterpup is lonely and is looking for someone to play with, but his attempts to do so unintentionally cause trouble for Luigi.

History
Polterpup first appears in Gloomy Manor, where he is playing in the Mudroom Exterior. He finds a jewel underground and plays with it shortly before getting it stuck in a tree. At this point, he leaves the Manor and becomes affected by the absence of the Dark Moon like the other ghosts within Evershade Valley.

Later, Polterpup appears in Haunted Towers after Luigi finds the special key in the pool, he runs up and takes it. However, unlike his fellow ghosts, Polterpup acts mischievous rather than hostile due to the lack of the Dark Moon's pacifying effects. This is made evident when Luigi briefly plays with him and, in the event Luigi loses all of his Heart Points, Polterpup will lick him back to health so long as Luigi has a Gold Bone. After Polterpup steals the special key, Luigi has no choice but to hunt him down in order 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.

Polterpup returns in the Old Clockworks, 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 again in the Treacherous Mansion. 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.

Polterpup is finally released from the container in the end after returning to a placid temperament thanks to the Dark Moon's effects. Unfortunately for him, without anyone to play with, the pup is unhappy. Despite Polterpup having caused his fair share of trouble, Luigi nevertheless cheers him up, much to Polterpup's delight. After Polterpup takes a photograph with the other ghosts, Luigi, Mario, and the Toad Assistants in celebration of defeating King Boo and restoring the Dark Moon, Polterpup accompanies Luigi back to his home as his new pet.

In-Game 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.''

Trivia

 * Polterpup is notably the only ghost creature that Luigi does not show any fear of in the game,. Instead, he will either be friendly toward it despite it being a ghost, or viewing it as a mere nuisance, as evident by Luigi saying "bad doggy!" whenever Polterpup ends up stealing important keys in both the Haunted Towers and the Treacherous Mansion.