The Ice King could also be a key figure. His constant attempts to kidnap princesses and his complex relationship with Finn could lead to a major plot point in the end. He might have a change of heart and end up helping Finn in some unexpected way, or his actions could be the catalyst for Finn's final transformation or decision.
Well, Finn himself is surely a key character. He's the protagonist and the story would likely revolve around his actions, decisions, and growth. Jake, his best friend, would also be crucial. Jake's unique abilities and their friendship are integral parts of Finn's journey.
Princess Bubblegum would be an important character too. Her scientific knowledge and her role as a leader in Ooo could play a significant part in the end of the fanfiction. Maybe she comes up with a plan to save Ooo or has some kind of revelation that affects Finn.
Obviously Finn himself is a central character. Jake, his best friend, is also very likely to be included. Princess Bubblegum often shows up as she has a complex relationship with Finn. Marceline might also be in there, as she is an important part of Finn's world and can play a role in his growth and maturity.
Another ending could involve Finn finally finding true love and settling down. He could choose to give up his adventuring ways and focus on building a family, bringing a new era of peace and stability to his part of Ooo. This would be a more heart - warming and domestic end to his story.
He might meet some of the more reclusive characters in Ooo that we don't usually see in the show. Maybe a hermit - like wizard who gives him some cryptic advice.
There could be other AI characters that are either friends or foes of Cortana. They might try to interfere with her control over the human or help her in some way. For example, another AI might think that Cortana's control is unethical and try to set the human free.
Obviously, Naruto would be one of the key characters. He's the main protagonist and his opinions often matter a great deal in the village. Also, Shikamaru, with his high intelligence, might be present and play an important role in the discussions.
Hinata would also be important. Given her love for Naruto, she would be at his side constantly, providing emotional support. And Kakashi, as Naruto's former teacher, might offer his wisdom and experience in dealing with difficult situations like this.
One of the main characters that a human female in equestria fanfiction might interact with is Princess Celestia. As the ruler of Equestria, she has a lot of wisdom and power. The human female could seek her guidance in understanding Equestria's politics and magic. Spike, Twilight's dragon assistant, is also a character that could be involved. He could be a source of comic relief in the interactions with the human female. Also, Discord, the spirit of chaos, could create some really interesting and challenging situations for the human female to deal with in the fanfiction.
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.
The key characters could be Harry's muggle relatives. The Dursleys have been his legal guardians in the muggle world for a long time. They might be fighting to keep Harry in their home. On the wizarding side, Hagrid could be important. He was the one who introduced Harry to the wizarding world. He might step in to ensure Harry is placed in a proper wizarding family or environment. Also, Hermione and Ron could play a role. They are Harry's best friends and would surely fight for Harry to be in a place where he can be happy and learn magic freely.
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.