Luna Lovegood could also be an interesting addition. The two could bond over their unique outlooks on magic. Luna might introduce Harry's sister to some of the more obscure magical creatures and concepts that she's discovered.
Hermione would be a great character to include. She could be like an older sister figure to Harry's little sister, helping her with her studies and giving her advice on how to handle the challenges at Hogwarts.
Draco Malfoy could be included in a more complex way. Maybe at first he teases and taunts Harry's sister, but as the story progresses, they find common ground, perhaps when they both realize they have to fight against a common dark force that threatens Hogwarts and the wizarding world.
Ron could also play an important role. He could be a bit jealous at first of the close bond between Harry and his sister, but then they become friends. Ron's sense of humor and loyalty would make him a fun addition. He could be involved in their adventures, like when they go looking for Horcruxes, and provide some comical relief during tense times.
If it's a crossover with 'Frozen', Elsa and Anna would be great characters. Ariel could meet them when a magical iceberg drifts into her underwater world. Elsa's ice powers and Ariel's water powers could interact in interesting ways.
Well, obviously the little sister herself is a very important character. Depending on how she's written, she could be really kind - like a Hufflepuff at heart - or really brave and adventurous like a Gryffindor. Then there are the classic characters from the original series that often play a big role. Hermione would likely be there to offer her wisdom and help the little sister with her studies. Ron would be the comical friend, always getting into some sort of trouble but also being very loyal.
Definitely the other ninja characters like Lloyd, Kai, and Jay. Their different personalities and powers would add depth to the story. Also, the villains from the Hands of Time series could be included to create conflict. For example, Acronix and Krux could be constantly trying to stop the sister from achieving her goals.
Hermione would be a great addition. Her thirst for knowledge and love of books would make her very interested in alchemy. She could help Harry decipher the complex alchemical texts. Also, Professor Snape could be involved. Given his expertise in potions, he might have some knowledge or interest in alchemy that could either help or hinder Harry's alchemical journey.
The main characters are Harry, who is the focus as Sam and Dean's sister. Sam, who is good at research and understanding the deeper aspects of the supernatural. And Dean, who is more hands - on and ready to fight. Their different personalities and how they interact with Harry form the core of the story in this fanfiction.
Hermione Granger could be involved. She's smart and might take an interest in how the Muggle government operates. She could help the Prime Minister understand the wizarding world better. Another character could be Arthur Weasley. His fascination with Muggle objects would make for some humorous interactions with the Prime Minister and also help bridge the gap between the two worlds.
Hermione Granger would also be a great addition. She could use her extensive knowledge from the library to help solve problems in the Herbology plot. Maybe she discovers a hidden section in the library about a lost herbology technique and tries to bring it back to the modern - day Hogwarts Herbology class.
If it's a crossover with 'Ice Age', Manny the mammoth would be a great addition. His calm and gentle nature could contrast with Grug's over - protective and sometimes gruff attitude. Sid the sloth could also bring some comical relief with his goofy antics.
Voldemort's remaining followers could be antagonists. They might target Harry's little sister thinking they can use her to get to Harry. They could try to kidnap her or use dark magic on her.