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.
Well, one interesting plot could be Clary slowly regaining her memories through a series of magical encounters. Maybe she finds an old object that triggers flashes of her past. Another could be that her friends play a crucial role in helping her get her memories back, like they take her to special places related to her past. And a third possibility is that there's some sort of enemy who is trying to stop her from getting her memories back, which adds a lot of suspense to the story.
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.
Obviously, Clary herself is central. Then there's Jace, her love interest. He would be very involved, probably being there for her throughout the pregnancy and during the birth. Simon might also be there. As Clary's best friend, he would offer his support in his own way, perhaps with some comical moments to lighten the mood.
Well, often it's Clary, Jace, and Simon. Simon is Clary's best friend, and in some fanfictions, his relationship with Clary can make Jace a bit jealous, but at the same time, if Simon seems too friendly with another girl, Clary might also get jealous in a different way. It's an interesting triangle - like situation that can add a lot of drama to the fanfiction. And of course, Magnus Bane might also make an appearance and his flamboyant personality could somehow be involved in situations that lead to Clary's jealousy.
Kakashi could also be an important character. He has a lot of experience and wisdom. When Naruto gets high, Kakashi might be the one to guide Naruto through this strange state, using his own past experiences and knowledge of different ninja techniques. And then there's Jiraiya. Given his relationship with Naruto, he would surely be involved, perhaps trying to use his own unique jutsu to help Naruto come back to normal.
Hinata would also play a major role. Since she has deep feelings for Naruto, she would be devastated by his amnesia. She might try to use their shared memories, like the time she confessed her love to him during the Pain attack, to jog his memory. And the Akatsuki members could also be involved in some cases. If Naruto's amnesia was caused by their actions, they might be the antagonists in the fanfiction, or they could also be trying to use his amnesia for their own purposes.
Obviously Naruto himself is the central character since the story is about him losing his memories. Sakura would likely be involved too. She's one of Naruto's closest friends and a medic - ninja, so she might try to find a way to restore his memories.
Obviously, Harry Potter himself would be the central character. He's the one getting the tattoos after all.
The villains in the Harry Potter series, such as Voldemort or his Death Eaters, could also play important roles. Harry's anger might be directly related to their actions. For instance, if Voldemort were to target someone Harry cares about again, it could be the trigger for his extreme anger. His interactions with these villains in the fanfiction would be full of tension and high - stakes confrontations.
In addition to the above, other characters from the wizarding world could be involved. For example, the Order of the Phoenix members. They might be suspicious of Harry at first but then come to his aid. And there could be some new characters created specifically for the fanfiction. Maybe a mysterious old wizard who has knowledge about the Dark Mark that no one else does and helps Harry on his journey to rid himself of it or at least understand it better.