There could also be some new characters introduced in the fanfiction. Maybe a mysterious unicorn with ancient knowledge about Nightmare Moon's origins. Or a group of creatures that have been affected by Nightmare Moon's previous reigns and are now seeking revenge or redemption by helping the ponies stop her third return.
Well, perhaps she comes back more powerful than ever. She might have new dark magic at her disposal and target different parts of Equestria this time.
Obviously, Ash would be one of the main characters. His Pikachu would also be there as his constant companion. Serena might also be a main character as she has a connection to Ash and is from Kalos.
I haven't read this specific fanfic in detail. But generally, in fanfics about Nightmare Moon's return in My Little Pony, they often explore themes like redemption, the struggle between good and evil, and the impact on Equestria. It might show how the ponies deal with this new threat and perhaps how Nightmare Moon's character develops during her return.
Obviously Sakura herself is the most important character. Naruto and Sasuke would also be important as they are her friends and comrades. Kakashi could also play a role, maybe as a mentor or someone who gives her advice on how to readjust to Konoha life.
There are several key characters in the My Little Pony Nightmare Moon story. Firstly, Twilight Sparkle, a magic - using pony with a thirst for knowledge. Her friends are crucial too. Pinkie Pie, the fun - loving and party - planning pony; Rainbow Dash, the fast - flying and brave one; Applejack, the hard - working and honest pony; Rarity, the fashionable and creative pony; Fluttershy, the shy but kind - hearted pony. And then there are Princess Celestia and Nightmare Moon (Princess Luna). Celestia has a connection to the whole situation as Luna's sister, and Nightmare Moon is the antagonist that the others must face.
The fanfic could be unique in how it weaves the plot. Maybe it combines different story arcs from the My Little Pony universe in a new way. For instance, incorporating elements from different seasons or specials to create a fresh narrative around Nightmare Moon's return. Also, the use of unique writing styles, like a more poetic or dark tone, can set it apart from other fanfics.
Perhaps there's also a mentor - like character. This could be someone who has had past experiences with the Nightmare Man and provides guidance to the protagonist, sharing secrets about the Nightmare Man's weaknesses and the nature of his existence.
Obviously, Greninja is one of the main characters as the title suggests its return. The trainer who was associated with Greninja before would also be a main character. They are likely to be central to the story's development.
There could be a new character that is the focus of the 'beautiful nightmare'. For instance, a young hobbit who discovers a hidden and dangerous power in the Shire. But the original characters like Merry and Pippin might also be involved, guiding or helping this new character through the nightmare situation while also dealing with their own fears and challenges within the LOTR world.
Since it's a fanfiction about an orphan in Naruto, one of the main characters is likely the orphan themselves. It could be an original character created by the fanfiction author. There might also be some Naruto - universe regulars like Naruto himself, as he was also an orphan and could potentially play a role in the story, either as a mentor or a friend to the new orphan character.