There could be the Guardians. They might play a role in the story. For instance, they could be initially hesitant about Jack Frost's sister being around, fearing she might disrupt the balance. But as they get to know her, they become friends. North could be there, with his jolly self, giving the sister a little magical gift.
In this fanfiction, it might also be about Jack Frost finally reuniting with his sister after a long time. He could be excited to share all his adventures with her. They might spend time reminiscing about their past, and then Jack could teach his sister how to skate on the frozen ponds he creates. There could be some comical moments too, like when his sister tries to make her first snowman and it turns out all wonky.
His parents could be in his family. Maybe his father is an ancient frost giant with great power over ice and snow, and his mother could be a gentle spirit who gives life to the frozen landscapes. There might also be siblings, like a sister who can create beautiful ice flowers.
The plot could involve Jack Frost initially being shocked to find his family. He may have thought he was the only one of his kind. His family might have been looking for him all this time. They could introduce him to the family traditions and ways of using their frost powers. And together they might go on an adventure to save their winter realm from some sort of threat.
In this fanfiction, Elsa and Jack Frost are the main characters. Elsa is a complex character, initially feared in her own kingdom because of her powers. Jack Frost, on the other hand, is a more free - spirited character who loves to play with children and spread winter magic. Their meeting in the fanfiction likely brings out new aspects of their personalities as they interact.
The main characters are obviously Jack Frost and the Guardians. The Guardians typically include characters like North (Santa Claus), Tooth (the Tooth Fairy), Bunny (the Easter Bunny), and Sandy (the Sandman).
Thor might be a great ally for Jack Frost. Thor is used to dealing with magic and powerful beings from different realms. He would be less likely to be intimidated by Jack Frost's powers. They could both use their elemental - related powers (Thor with lightning and Jack Frost with ice) to create powerful combined attacks against their enemies. For example, Thor could strike his lightning into Jack Frost's ice structures, creating a powerful blast of energy.
Well, in such a fanfiction, Jack Frost could bring his icy powers to the Avengers. Maybe he'd help them fight against a new villain who has a weakness to cold. He could freeze the bad guy's weapons or create ice barriers for the Avengers to take cover behind.
One possible plot could be that Jack Frost, being the spirit of winter himself, is intrigued by Elsa's ice powers. They could start by having a friendly competition to see who can create the most magnificent ice sculptures. Their powers combined might accidentally create a new, magical ice realm that they then have to explore and protect together from an approaching threat, like a dark sorcerer who wants to steal their ice magic.
Obviously, Jack Frost is the main character as the story is about his transformation into a dark version of himself. But there could also be other characters from the original Jack Frost lore, such as the Guardians like North, Bunny, and Tooth. They would likely play important roles, either trying to save Jack from his dark path or being forced to oppose him if he becomes too dangerous.
The main characters are obviously Jack Frost and Baby Tooth. Jack Frost is often depicted as a fun - loving, mischievous spirit of winter. He has the power to control ice and snow. Baby Tooth, on the other hand, is a tooth fairy. She is very small and cute and has a big role in collecting teeth from children. In fanfiction, they usually interact in various ways, like going on adventures or just having friendly chats.