Well, of course, the main character is 'him'. But there could also be other important characters like his mentor who guides him on his journey. This mentor could be an old and wise figure who imparts valuable knowledge.
Often, his friends or comrades can be main characters in the fanfic. They might have different abilities and personalities. For example, there could be a brave and impulsive friend who often gets into trouble, and 'him' has to bail him out while also learning important lessons about friendship.
The main character is surely Harry. However, in a fanfic about uniting all, Dumbledore could play a significant role as a mentor figure, guiding Harry in his quest. Neville might also be an important character as he represents the growth and strength of the students at Hogwarts. Additionally, characters from different magical species, like a centaur or a goblin, could be main characters if the fanfic is about uniting the entire wizarding community which includes all these different elements.
A likely main character could be a young ninja who discovers he has the 'One for All' power. He would be on a journey to understand and control it, while also dealing with the normal challenges of being a ninja in the Naruto world, like training and making friends.
A great 'all for one all for him fanfic' might revolve around his relationships. For example, how he forms strong bonds with his allies, and how those relationships help him grow as a character. He could start off as a lone wolf but through these connections, become part of a powerful team.
There's no way to tell for sure. It might be a patient as one of the main characters who is experiencing this dream - like state in the hospital. There could also be a doctor or a nurse who plays a crucial role in the story, either helping the patient understand the dream or being part of the mystery within the dream.
Well, obviously baby All Might is a main character. He's likely to be the center of the story, with his cute and innocent personality.
Obviously, Izuku is the main character. He's the one who is mastering One For All. All Might also often appears as he was the previous user and is Izuku's mentor in many fanfictions. Bakugo can also be a significant character as he serves as both a rival and sometimes an unexpected ally to Izuku in his journey with One For All.
Charlotte's family members could also be main characters. Her parents, for instance, who have influenced her upbringing and continue to be part of her life as she grows. They might be shown guiding her through different life decisions or having their own story arcs that intersect with Charlotte's.
Naruto is likely to be one of the main characters as it's related to the Naruto universe. Since it's about the creation of all things, he might be the one exploring or using this power.
I'm not sure specifically as I haven't read it. But it could be the main characters from the original work on which the fanfic is based. For example, if it's a fanfic of a TV show about a group of friends, the main characters would likely be those friends, and the fanfic would explore their love relationships in new ways.
I don't know as I haven't read it. But likely it includes characters from the Twilight series such as Bella, Edward, and Jacob.