Polterpup

Polterpup (known as ??? in E. Gadd's Vault) is a specific Polterpup character who appears throughout Luigi's Mansion: Dark Moon. Unlike normal Polterpups, he has a red collar instead of blue. His name, Polterpup, comes from a combination of "Poltergeist" and "Pup". E. Gadd surmises that Polterpup is lonely and is looking for someone to play with, but his attempts to do so accidentally 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.

Later, Polterpup appears in Haunted Towers after Luigi finds the key in the pool, he runs up and takes it. He plays with it, similarly to what he did with the jewel, and throws it out the window. He chases after it, and is not seen until the next level, where Luigi hunts him down. Luigi finally catches him, but before he can store him in the Ghost Container, the pup managed 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; unfortunately, the pup still manages to escape 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, E. Gadd manages to have fixed the Ghost Container, meaning that the Polterpup can't escape.

Polterpup is finally released from the container in the end, after turning normal. Unfortunately for him, without anyone to play with, the pup is unhappy, but Luigi cheers him up, and after Polterpup has taken a photograph with the other ghosts, Luigi, Mario, and the Toad Assistants, Polterpup returns home as a new pet.

Also, whenever Luigi loses all of his health, if he has a Gold Bone with him, Polterpup will come up to him and lick him back to health, even if the dog is trapped within the Ghost Container.

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 doesn't show any fear of in the game, at best being friendly to it despite it being a ghost, and at worst viewing it as a mere nuisance (as especially evident by his saying "bad doggy!" when Polterpup ends up stealing important keys in both the Haunted Towers and the Treacherous Mansion).