Finn is the adventurous and somewhat naive human. His perspective is always interesting as he views the magical world of Ooo through the eyes of an outsider. The lady, whoever she is, usually has her own set of values and goals. If it's Flame Princess, for example, she has her power and her own kingdom to think about. Their conversations in the fanfiction are shaped by these aspects of their characters.
One interesting storyline could be Finn and the lady having a deep conversation about the nature of good and evil in the Land of Ooo. Maybe they discuss how to handle the various villains they've encountered.
Obviously, Finn and the Jungle Princess are the main characters. Finn, a brave and adventurous boy, and the Jungle Princess, who has her own set of skills and knowledge about the jungle environment.
The main characters are Finn, a brave and adventurous human boy, and the Water Princess. Finn is known for his heroics and his sword - fighting skills. The Water Princess has control over water and is from the Water Kingdom.
Princess Bubblegum could be a part of the fanfiction. She might be interested in the unique flora and fauna of the Canyon and could send Finn on a mission there. Marceline the Vampire Queen could also be added. Maybe she has some history with the Canyon and her presence could add an element of mystery and danger.
The main characters are obviously Finn and Huntress Wizard. Finn, a brave and adventurous human boy, is known for his heroics in the Land of Ooo. Huntress Wizard, on the other hand, is a mysterious and powerful magic - user.
Obviously, Finn in his dark form is the central character. Jake often appears as well, either trying to help Finn or being conflicted about his now - dark friend. Princess Bubblegum might also be involved, using her scientific knowledge to try and find a cure for Finn.
Princess Bubblegum could also be a main character in such fanfictions. She might be the one trying to find a scientific solution to Finn's vampirism. Her intelligence and resources in the Candy Kingdom could play a big role in the story. Another character could be Marceline. As a half - vampire herself, she could either be a mentor to Finn or a source of conflict as she has her own views on vampirism.
Another possible theme is the exploration of different cultures within the Adventure Time universe. Lady Rainicorn comes from a different species, and fanfictions can delve into how her culture and Finn's human - like culture interact and influence each other.
Obviously, Jake would be one of the main characters Si, reborn as Finn, would interact with. They would go on all sorts of wacky adventures together. Also, Princess Bubblegum. Since Finn often helps her with various problems in Ooo, Si would also be involved in those situations, like dealing with the Candy Kingdom's threats.
The Fire Kingdom's Flame Princess could be an ally too. Her mastery over fire could complement Finn's powers in combat situations. Also, the various magical creatures in Ooo, like the gnomes or the fairies, could band together and support Finn. They might have their own ancient knowledge or magic that could be useful to Finn in his adventures with his godlike powers. Finn's kindness towards them in the past might make them more inclined to help him out.