Eleven herself is the central character. Her friends like Mike, Dustin, and Lucas would likely be involved. Mike, being her close friend and having a special bond with her, would be there for her. Dustin and Lucas would probably offer their own unique forms of support.
Hopper, as Eleven's guardian, would be part of the story. He might be a bit clueless at first but would do his best to handle the situation. Also, the other female characters in the Stranger Things world, even if they have smaller roles, could contribute to Eleven's experience, like Nancy or Karen Wheeler offering some words of wisdom from their own experiences.
Obviously Caroline herself is the main character. Her mother is likely to be in it as well, as mothers often play an important role in a girl's first period experience, like giving advice and support.
Clary is the central figure. Valentine might also be involved in some fanfictions, either as an obstacle trying to prevent Clary from getting her memories back because he doesn't want her to discover certain secrets, or in a more complex way where his past actions are somehow related to the loss of her memories in the first place. And of course, the Lightwood family members are likely to be involved, given their importance in Clary's life and the Shadowhunter community.
Perhaps it focuses on Eleven's own journey of self - discovery during this time. She might feel different and start to explore more about her femininity. It could also show how she copes with the hormonal changes and the emotions that come with getting her period.
Toriel is very likely to be involved. She has a motherly nature and would surely be concerned about Frisk's health. She would probably be at Frisk's bedside, offering comfort and warm food.
Obviously Naruto himself is the main character involved. But his friends like Sasuke, Sakura, and Kakashi are also likely to be involved. Sasuke might be affected by Naruto's wishes, for example, if Naruto wishes for Sasuke to fully return to the Hidden Leaf Village and be at peace. Sakura could also be involved, perhaps in helping Naruto make his wishes come true in a medical or support way. And Kakashi, as Naruto's former teacher, would surely play a role in guiding Naruto in how to use his wishes wisely.
Obviously, Dumbledore himself is the central figure. Then there would be characters like Harry Potter, who might have mixed feelings about Dumbledore after learning some of his secrets. Hermione Granger could also be involved, using her logical mind to analyze the situation. And perhaps members of the wizarding government, such as the Ministry of Magic officials who might be the ones to judge Dumbledore.
In addition to the ones mentioned, Dumbledore could be a character in the fanfiction. He might try to temper Sirius's thirst for revenge and guide him towards a more just path. On the enemy side, Peter Pettigrew would be a central figure. Since he was the one who really betrayed James and framed Sirius, Sirius would likely be out for revenge against him. And Bellatrix Lestrange, as a prominent Death Eater, would also be a likely target for Sirius's revenge.
Definitely his friends from Camp Half - Blood. Percy Jackson, for instance, would be likely to be involved. He might be the one to find Nico when he's hurt or be part of the group that's trying to rescue him. Annabeth could also be involved as she is often in the thick of things and would use her strategic mind to help Nico when he's injured. Hazel, being his sister, is very likely to be there. She would be extremely worried and do whatever she can to heal or protect him.
Rukia could also be in the fanfiction. Given their close relationship, she might be the first one to notice something is off with Ichigo. She could be the one trying to keep him out of trouble or scolding him for getting drunk in the first place.
Obviously, McGee is the central character as he's the one getting hurt. Gibbs would surely be involved. He'd be the one leading the investigation into McGee's injury, using his no - nonsense approach. DiNozzo would also be there, probably cracking some inappropriate jokes at first to lighten the mood but then getting serious as the situation unfolds.