Dan and Amy's parents, though not always present in the series, could be key characters in these fanfictions. Their absence or the mystery around them could be related to Dan's adoption. Maybe his biological parents were somehow connected to the '39 Clues' mystery. Also, any new characters introduced specifically for the fanfiction, like a wise old mentor who knew Dan's birth parents and guides him on his journey to discover his past, could also be main characters in these stories.
There might be a fanfiction where Dan, after finding out he's adopted, has to deal with the emotional turmoil within his family. His sister Amy could be really supportive, but there could be other relatives who react differently. The story could explore how this new knowledge changes his relationships and his role within the family in the context of the '39 Clues' world.
Obviously, the priestess herself is a main character. She is often depicted as having unique spiritual powers and a mysterious background. Naruto also frequently appears, as his story is intertwined with the events in the Naruto universe. Sakura could also be involved, perhaps as a friend or an ally to the priestess, using her medical skills to support the priestess in her quests.
The main character is Percy, of course. But in addition to his close friends, some fanfictions might also involve the gods. For instance, Poseidon, Percy's father, could make an appearance. He might be worried about his son's condition and use his godly powers to try and save Percy. Or Chiron, the centaur trainer at Camp Half - Blood, would also be a key figure. He has a vast knowledge of demigod health and history, and would be guiding the others in their efforts to deal with Percy's collapse.
Obviously, Tony Stark himself is the central character as it's about his collapse. But other key characters usually include Pepper Potts, his love interest. She often plays a role in either trying to prevent the collapse or dealing with the aftermath. For example, in some fanfictions, she's the one who discovers him first.
Obviously, Bella's daughter is the central character. Often, Edward Cullen, as the father, is also very involved. He can help guide their daughter in understanding her vampire side. Jacob Black might also be a significant character, perhaps having a protective or friendly relationship with Bella's daughter, similar to his relationship with Bella in the original series.
The main character is Harry Potter without a doubt. However, new characters can also be introduced in these fanfictions. For example, there could be a local wizard or witch in the place where Harry is exiled who becomes his friend or mentor. This new character can have their own backstory and motives. They could teach Harry about the local magic or help him adapt to his new environment, adding a new dimension to the story.
Naruto's friends like Shikamaru and Choji could also be part of 'Naruto Controls Heat' fanfictions. Shikamaru might use his intelligence to come up with strategies that incorporate Naruto's heat - controlling ability. Choji, on the other hand, could be a source of support and a test subject in some cases, for example, to see if Naruto can control the heat in a way that doesn't harm Choji when they are training together.
Naruto is often a main character in these fanfictions. Since he's the protagonist of the original series, it's natural for him to be involved in stories about new techniques like the nerve strike. Sasuke is also a common character. His skills and rivalry with Naruto can be incorporated into stories where the nerve strike becomes a factor in their battles or their growth as ninjas.
Well, Naruto himself is obviously the main character as it's about him as the Orange Hokage. His friends like Sasuke might also be important characters. Sasuke could be shown either supporting Naruto's new role or having some differences in opinion on how Naruto should lead as the Hokage.
Besides the main trio, characters like Kakashi could be involved. He might be the one who secretly has a sweet tooth and joins Naruto in trying out new ice cream flavors. Also, characters like Shikamaru, known for his laid - back nature, could be part of the story. He could be the one who comes up with clever ideas for ice cream - related events in the village. And of course, the local ice cream vendors in the Hidden Leaf Village would be important characters as well.