A merman could be one. He might be curious about the Black Sea Reaper and try to make friends or, on the contrary, see it as a threat to his underwater territory. Another could be a sea witch. She could have her own motives, either to help or to hinder the Black Sea Reaper. A young sea turtle could also be a character. It could look up to the Black Sea Reaper as a hero and follow it around on its adventures.
Potential characters include a sea nymph. Sea nymphs are often associated with the beauty and magic of the sea. The Black Sea Reaper might be attracted to her beauty or be in need of her magic powers for a particular task. A kraken could also be a character. The kraken is a fearsome and powerful creature. It could be an enemy of the Black Sea Reaper, constantly challenging it for dominance in the Black Sea. Additionally, a selkie could be part of the fanfiction. Selkies are known for their ability to transform between human and seal forms. They could bring a different perspective to the story as they have a connection to both the land and the sea, and might interact with the Black Sea Reaper in unique ways.
There are several potential characters. For instance, an old and wise octopus. This octopus could have a lot of knowledge about the secrets of the Black Sea and could either share that knowledge with the Black Sea Reaper or use it against it. A group of dolphins could also interact with the Black Sea Reaper. They are known for their playfulness and intelligence, and they could either play pranks on the Black Sea Reaper or help it in a time of need. And then there's the possibility of a human sailor who somehow ends up in the Black Sea and has an encounter with the Black Sea Reaper. This could lead to a whole series of events as the sailor tries to understand and survive the encounter.
Obviously, his old friends like James Potter or Remus Lupin could come to visit him in the hospital. They would bring some of their old Hogwarts memories and cheer him up. Then there could be the hospital staff, like a strict but kind - hearted matron who takes care of Sirius. Also, other patients in the hospital might interact with him, such as an old wizard who has some valuable advice for Sirius.
You could add a young and naive ghoul who looks up to the Black Reaper as a mentor. This ghoul could be new to the world of ghouls and need guidance from the Black Reaper. Another potential character could be a mysterious old ghoul with vast knowledge of forgotten ghoul techniques. He could be an ally or an adversary depending on the situation. And a third character could be a human with psychic abilities who can sense the presence of ghouls. This human could befriend the Black Reaper or try to hunt him down.
There could be new characters as well, like a devil mentor figure who teaches Percy about his devil heritage. This character could be ancient and have a lot of knowledge about the devil realm. Or there could be an angelic character who is initially an enemy but later realizes Percy is not fully evil and they end up forming an unlikely alliance.
Naruto might interact with Eostia's warriors. They could train together, Naruto showing them his ninja combat skills and them teaching him about Eostia - style combat. There could also be some mysterious characters in Eostia, like an old hermit who has some knowledge about Naruto's sudden appearance in their world and could guide him on his journey in Eostia.
Sasuke could be one. He might be intrigued or even challenged by Naruto's new wild ways. Their rivalry could take on a new dimension as Sasuke tries to understand what has changed Naruto so much.
Obviously, Buffy would be a major character he'd interact with. They could start off as wary of each other but then grow to respect one another's fighting skills. Giles too, as he has a wealth of knowledge about the supernatural, and he and Harry could have some interesting discussions about different types of magic.
In a crossover with the Attack on Titan world, characters like Eren could interact with Naruto. Eren, with his strong will to fight for freedom, could learn from Naruto's experience as a father and a hero. Levi might also be interested in Naruto's combat skills and how he trains his child in the ways of the ninja.
He could interact with Mei Terumi, the Mizukage. She might be initially cautious of Naruto but then see his potential and work with him. Also, Chojuro, as he is one of the important ninjas in Kiri. They could train together or go on missions side by side.
Well, Kakashi could play an important role. Since he was Naruto's mentor, he would be very curious about Naruto's sudden change to a nobody. He might secretly investigate what caused this. Shikamaru could also interact with Naruto. Shikamaru is smart and observant. He might notice some small details about Naruto that others miss, and try to figure out the mystery behind Naruto becoming a nobody. Also, Rock Lee could be in the story. Lee might see Naruto as a new opportunity to spread his 'youth' spirit, not knowing Naruto's past.
Sakura could have interactions with the Weasley family. Ginny Weasley might become her friend and they could have girl - talk about magic and boys. Fred and George could also be part of her story. They might play some pranks on her at first but then become good friends. And don't forget Neville Longbottom. Sakura could help Neville gain more confidence in his magic abilities as they study together in the Hogwarts library.