Who could be some unique characters to include in Luigi's Mansion 3 fanfiction?

A talking portrait that gives Luigi hints and clues. It could be a former owner of the mansion who is now trapped in the form of a portrait. Another interesting character could be a ghost dog that befriends Luigi and helps him sniff out hidden items or ghosts.

One idea could be exploring Luigi's character growth. Maybe he discovers a new power within the mansion that helps him face his fears more bravely. Another idea is to create a backstory for the ghosts in the mansion, like how they came to be there and what their lives were like before. A third idea could be a crossover where characters from another game enter Luigi's Mansion, creating new and exciting adventures.

Luigi is the central character in the Luigi's Mansion Dark Moon story. He is the one who is called upon to solve the mystery of the shattered Dark Moon. There are also Professor E. Gadd who provides Luigi with the tools like the Poltergust to help him in his quest. And of course, there are numerous ghosts which range from common ones to more menacing boss ghosts. These ghosts are the ones that Luigi has to capture in order to complete his mission.

Mario could be a great addition. Since Luigi is Mario's brother, his presence could add some family - related drama or comical moments. Sonic could also be included. As Tails' friend, Sonic's brash and confident personality would contrast well with Luigi's more timid nature.

In this fanfiction, Luigi might be exploring the mansion when he stumbles upon Tails. Tails, being the tech - savvy character he is, could help Luigi figure out the strange mechanisms in the mansion. Maybe they'll find hidden passages together and encounter some spooky but friendly ghosts.

You could include Sherlock Holmes. His deductive skills would be very useful in trying to figure out the mysteries of the haunted mansion. He could analyze the strange happenings and the behavior of the ghosts and spirits, perhaps even finding a logical explanation for some of the seemingly supernatural events.

Princess Peach would be a great addition. She could be in trouble as usual, and Mario and Luigi have to rescue her. But this time, she could also play an important role in helping them solve a mystery or defeat a powerful enemy. For example, she might have some knowledge about an ancient artifact that the brothers need.

For a Luigi's Mansion and Super Smash Bros. crossover fanfiction, Luigi could be in the middle of a Smash Bros. battle when a portal to his mansion opens up. Characters from the Smash Bros. roster get accidentally sucked in. Luigi has to lead them through his mansion, fighting off the more dangerous ghosts. Some characters like Kirby might try to eat the ghosts, while others like Link would use their weapons. Along the journey, they discover secrets about Luigi's Mansion that Luigi himself didn't know, all while trying to find a way back to the Smash Bros. arena.

A plot could revolve around Luigi having to rescue all the Toads that have been scattered throughout the mansion in different forms - some are invisible, some are trapped in paintings. As he searches for them, he uncovers secrets about the mansion's original owner and their dark past.

Mario would definitely be a main character. Since Luigi is his brother, he would be the one most concerned about Luigi's transformation and would do everything he could to help him. Princess Peach could also be a main character. She might offer her castle as a safe place for baby Luigi and use her resources to find a solution. Toad could be there too, helping Mario with taking care of Luigi and providing useful information about the kingdom that could lead to reversing the spell.