Well, Renesmee is at the heart of it all. Then there are her loving parents, Bella and Edward. Jacob, the werewolf, is also a key player given his special relationship with Renesmee. The Volturi, being the ones who take her, are all important characters. Aro, with his power of seeing others' thoughts through touch, is a major force. Caius, who is often more aggressive in the Volturi, and Marcus, who can sense relationships. And don't forget Carlisle, the patriarch of the Cullen family, who may try to use his diplomatic skills to deal with the Volturi and get Renesmee back.
The key characters would be Renesmee herself, since she is the one who has been taken. Her parents, Bella and Edward, are bound to be central in trying to rescue her. Jacob, with his unique bond to Renesmee, would also be very important. Among the Volturi, Aro is a key figure as he usually makes the major decisions. And other members of the Cullen family like Alice, who can see the future, and Jasper, who can influence emotions, might also play crucial roles in either rescuing Renesmee or dealing with the Volturi.
Definitely the Cullens, especially Bella and Edward as Renesmee's parents. Also, Jacob who has a strong connection to Renesmee. And of course, the Volturi leaders like Aro, Caius and Marcus.
In such a fanfiction scenario, first of all, the Volturi would likely have their reasons for taking Renesmee, perhaps believing she is a threat to the vampire world order. Once taken, the Cullens would be desperate. Alice might use her visions to try to find Renesmee's location. Jacob could also play a crucial role as he has a special bond with Renesmee. He may attempt to lead a rescue mission or at least try to communicate with her while she is in the Volturi's custody. The story would be filled with drama, action, and the exploration of relationships between different characters in the 'Twilight' universe.
Edward and Bella could be important characters. As Renesmee's parents, they would likely be deeply involved in the situation. They might be worried about her well - being, try to protect her from any threats related to the pregnancy, and offer their support and advice.
Obviously, Sonic and Eggman are the key characters. Sonic is the kidnapped one and Eggman is the kidnapper. But also, Tails could be a very important character. Since he is Sonic's best friend, he would likely be the first to try and rescue Sonic. Knuckles might also be involved. He could help in the search or be the muscle during the rescue mission.
Maybe a human who has discovered the existence of vampires and wants to use Renesmee to prove it to the world. They might think that by kidnapping her, they can expose the vampire world and gain fame or fortune. Another option is a rogue werewolf who doesn't like the alliance between the vampires and werewolves and decides to take Renesmee as a way to break that bond.
Miu could be a key character. She might be the one who stays by Kenichi's side constantly, using her skills to keep him as comfortable as possible while he's sick.
Harry is the central figure in need of help. Queen Elizabeth (assuming she is still part of the story's context) could be a key character. Her wisdom and authority within the royal family could be crucial in providing the kind of help Harry needs. Also, Meghan Markle could be involved. As Harry's wife, she might be part of the situation, either being in need of help along with Harry or playing a role in how the royal family's help is received and utilized.
Dumbledore could also be a key figure. He has so much knowledge and wisdom that he might know some ancient magic or spell that could cure Harry. He could guide Harry through the process of getting his health fixed and offer moral support as well.
Obviously, Harry Potter himself is the central character as the story is about him getting top owl results. Hermione would also be important as she is always the one to help Harry with his studies.
In this 'jace gets sick fanfiction', Jace is the one who gets sick, of course. But there might also be his family members. For example, his mother could be a very important character, hovering over him, making sure he takes his medicine and gets enough rest. And perhaps there is a wise old doctor in the story, who tries different remedies to cure Jace.
Definitely Aro. He is the leader of the Volturi, so he will have a lot to do with Bella.