Professor Dumbledore is a character that is sometimes part of these crossovers. His wisdom and power in the wizarding world could be a great counterpoint to Ben's more action - oriented heroics. He could perhaps guide Ben in understanding the more complex aspects of the wizarding world. And then there's Vilgax from the Ben 10 universe. Writers sometimes bring him into the Harry Potter world as a new and powerful enemy for both Harry and Ben to face together.
Harry Potter and Ben Tennyson are obviously very popular. Harry's role as the hero in the wizarding world and Ben's as the wielder of the Omnitrix make them natural choices for crossovers. Hermione is also often included as her intelligence could be used to figure out the science behind the Omnitrix.
Ron Weasley is another character that often shows up in these crossovers. His comical nature and loyalty can add an interesting dynamic. On the Ben 10 side, Gwen Tennyson is popular too. Her magical abilities in the Ben 10 universe could interact in interesting ways with the magic of Harry Potter. For example, she could try to compare and contrast her spells with the Hogwarts magic.
Elsa is quite popular. Her ice powers make for an interesting contrast with the magic in Harry Potter. Hermione is also a common character to cross over. Her intelligence and love for knowledge would make her curious about the world of Frozen 2. Harry Potter himself is often used in crossovers as well, as his heroic nature can fit into the story of Frozen 2 easily.
Harry Potter and Link are two of the most popular characters to cross over. Harry, with his magical abilities and experience in fighting dark forces, and Link, the hero with his sword - fighting skills and courage. When combined, they can create some really exciting storylines.
Harry Potter himself is often a popular choice to cross over. His courage and magical abilities make him a great addition to the Narnian world. Also, Aslan from Chronicles of Narnia is sometimes brought into the Harry Potter universe. His powerful and wise nature can bring a new perspective to the wizarding world.
Hermione Granger is also very popular. Her intelligence and knowledge of magic would be valuable in Narnia. She could study the magic in Narnia and perhaps find ways to combine it with what she already knows from Hogwarts.
In 'harry potter flash crossover fanfiction', Harry Potter and the Flash are the obvious front - runners. But aside from them, characters like Sirius Black could be interesting. He has his own set of skills and a rather rebellious nature. Meeting the Flash could result in some exciting adventures. Ginny Weasley, with her courage and talent in Quidditch, might also be a character that fans like to cross over. She could potentially learn some new skills from the Flash or vice versa. And then there's Professor Snape. His complex personality and his skills in potions and the dark arts could create some intense and thought - provoking crossovers with the Flash.
Hermione is a prime candidate for an interesting crossover. In an anime world like My Hero Academia, she could use her intelligence and magic to attend U.A. High School. She'd be a unique student, using spells instead of quirks, and could help solve some of the complex problems in that world. Ron Weasley could also be fun. If he crossed over into the world of Inuyasha, he could bring his sense of humor and his skills with magic items. He might accidentally cause some chaos with his clumsy ways, but also be a valuable ally in the fight against Naraku. And then there's Luna Lovegood. In an anime like Spirited Away, her dreamy and eccentric nature could fit right in as she explores the strange spirit world, using her unique magic and knowledge of the wizarding world's odd creatures.
Harry Potter himself is often a popular choice. He can be thrown into the chaotic world of Warhammer and his skills and values are put to the test. Another is Dumbledore. His great wisdom and powerful magic could be an interesting match against the dark forces in Warhammer.
Hermione Granger and Hera Syndulla are often crossed over. Hermione's intelligence and vast knowledge of magic can be paired well with Hera's leadership and piloting skills. Also, Han Solo and Ron Weasley are popular choices. Their roguish charm and sense of humor are quite comparable, and fanfiction writers like to explore how they would get along or what kind of adventures they would have together.
Snape and Kylo Ren could be an unexpected but interesting crossover. Snape's complex character, his past mistakes and his hidden loyalties could be compared to Kylo Ren's inner turmoil. Snape might see some of his own struggles in Kylo Ren and try to either redeem him or use him for his own purposes. And Hagrid and Chewbacca would be a fun crossover. They are both big, lovable characters with a heart of gold, and seeing them interact would bring some light - hearted moments to the story.
Dumbledore and Galadriel. Both are very powerful and wise characters. Dumbledore with his deep understanding of magic and his ability to see the good in people, and Galadriel with her elven magic and her role as a protector. Their encounter could lead to some very interesting discussions about the nature of magic, good and evil, and how to deal with powerful dark forces in both their worlds.