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.
In the 'Harry Potter nightmare man fanfiction', the 'nightmare man' could be a figure born out of the dark magic that has seeped into the wizarding world over time. He might not be a traditional wizard but more of a shadowy being. His identity could be a mystery at first, and as the story unfolds, we learn that he was once a normal wizard who was corrupted by a powerful and evil force. He then uses his new - found dark abilities to torment the characters we know from the Harry Potter series.
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.
The main characters could be a new generation of ninja students in the Naruto world. They stumble upon the knowledge of 'Nightmare Release' and have to deal with the consequences. This could include a young ninja who has a natural affinity for this strange power and has to learn to control it while facing the challenges from other ninjas who fear or want to use this power for their own gain.
It really depends on the specific fanfiction. However, it could feature Naruto himself dealing with a character who has the nightmare bloodline and trying to understand their motives.
Since I don't know the exact 'nightmare man story', it's hard to say. But it's likely that the main character is the 'nightmare man' himself.
The main character in 'the nightmare man story' is likely the nightmare man. He is probably the central figure who drives the plot. He could be a menacing presence, and the story might focus on his actions, like how he torments the other characters in the story through their nightmares.
Often, the 'nightmare man' is an antagonist in the fanfiction. He could be a figure from the past, a forgotten wizard who was once defeated but has now returned with new powers related to nightmares. For instance, he might have the ability to make people relive their worst memories in their dreams, weakening them in the real world. This makes him a formidable foe for the heroes of the story.
Obviously Naruto himself would be a main character. He would be the one to bring his ninja skills and determination into this strange combination of worlds. Sasuke could also be there, maybe as a rival who has different ideas on how to deal with the Gundam - related threats. And from the Gundam side, Amuro Ray could be a key character. He might be the one who first discovers Naruto and his group and has to decide whether to help or oppose them.
Ichigo from Bleach could be a main character. His strong sense of justice and powerful combat abilities would make him a great fit for the nightmarish world of Little Nightmares. He could be on a quest to save the innocent souls trapped there.