Well, obviously Finn's sister herself is a popular character in these fanfictions. Also, Jake the dog might be involved as he is Finn's best friend. Princess Bubblegum could also be popular as she is an important figure in the Adventure Time world. She might interact with Finn's sister in various ways, like teaching her some science or asking for her help in a kingdom - related matter.
The Ice King can be a really interesting character in these fanfictions. He might have a different relationship with Finn's sister compared to his relationship with Finn. Maybe he sees her as a new potential princess to kidnap or he could actually befriend her in some strange way. Marceline the Vampire Queen is another one. She could be a mentor - like figure to Finn's sister, teaching her about the darker aspects of Ooo and how to survive them.
There could also be a fanfiction where Finn's sister goes on a journey to find out more about her family's past. Along the way, she meets different characters who knew her parents. This story could be full of emotional moments as she uncovers secrets about her family, and at the same time, has to deal with the dangers of the Ooo world.
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.
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.
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.
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.
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.