Harry Potter himself is often a main character. Since it's a fanfiction related to his world, he might be the one to inherit something from Merlin. For example, he could inherit Merlin's wand or a special magical ability that Merlin had.
Dumbledore is often a main character in Harry Potter 1945 fanfiction. Since he was around during that time, many stories explore his youth, his friendships, and his early battles against the dark forces.
In some cases, there might be new original characters created specifically for the fanfiction. These could be characters who have a connection to the ancient magic of Merlin and Morgana. For example, a mysterious wizard who has been guarding an artifact related to their power. They could initially be an enigma to Harry and his friends, but as the story progresses, they become an important part of the plot, either as an ally or an adversary.
Obviously, Merlin is the central character. His character is the focus as he returns and the story usually revolves around his actions, decisions, and how he impacts the world around him again.
Dumbledore can also be a main character in such fanfictions. He might have known more about the Lightning Bolt scar than he let on in the original series. Fanfiction could explore his past interactions with Harry's parents regarding the scar and how he was secretly guiding Harry towards unlocking the full potential of the Lightning Bolt.
Harry Potter himself is often a main character. After all, it's a Harry Potter fanfiction. He is likely to be the one facing the challenges related to the mist.
Well, Harry is almost always a key figure. He could be seen training under a samurai master in some fanfictions to gain new powers or a different perspective on magic. Hermione might be portrayed as the one who studies the samurai culture deeply, trying to find connections between their magic and the samurai way. Another character that often shows up is Dumbledore. He could be the one guiding the integration of the two cultures, using his wisdom to ensure harmony between the wizarding world and the samurai - inspired elements. There may also be a samurai - style villain, someone who challenges the values of both the Harry Potter world and the samurai code.
Harry Potter is the main focus in 'harry potter invincible fanfiction'. But other characters like Dumbledore can also be important. Dumbledore, with his great wisdom, might be guiding Harry in how to handle his invincibility or using Harry's power for the greater good of the wizarding world. Then there are the villains. Voldemort is likely to be a key figure as Harry's invincibility would change the dynamic of their fight.
In some 'in noctem Harry Potter fanfiction', Severus Snape can be a main character. His complex relationship with Harry and his own mysterious past can be explored in the context of the night. For example, a fanfiction could show Snape patrolling the castle at night and having an unexpected encounter with Harry that reveals a new side of their relationship.
Draco Malfoy could be a main character as well. Fanfictions might explore his academic rivalry with Hermione or other students. He was from a family that expected a lot from him in terms of academic achievements, so a story could revolve around him trying to outdo others in getting the best grades or facing some academic setbacks that change his perspective on grades and competition.
Severus Snape is often a key character in these fanfictions. His complex relationship with Harry can take on new dimensions when it comes to forsaken magic. He might have some hidden knowledge about it or be involved in a plot that involves the forsaken aspects of the wizarding world. And of course, Dumbledore might also play a role, either through his past actions related to the forsaken magic or through his posthumous influence on Harry.