There could be Firenze, the centaur. Centaurs are known for their wisdom about the forest and its creatures. He might be able to guide Harry to the unicorn and provide some herbal remedies or magical advice for the healing process. Also, Hermione could be involved. She would surely do a lot of research in the library to find the best way to heal the unicorn.
Obviously, Harry is one of the main characters. Since it's about him healing the unicorn. The unicorn itself can also be considered a main character as the story revolves around it. There might also be other characters like Hagrid. Hagrid could be the one who tells Harry about the injured unicorn in the first place, or he could assist Harry in some way during the healing process.
In a 'harry heals a unicorn fanfic', perhaps Harry uses his magic skills and knowledge. He might find an injured unicorn in the Forbidden Forest. He could use spells like 'Episkey' or some advanced healing magic he has learned. Then he has to deal with the unicorn's trust issues as they are very shy and skittish creatures.
Hermione could be there. She might be the one explaining the significance of Beltane to Harry. Ron could also be present, probably more interested in the food at the Beltane celebration.
There could be other Hogwarts students. For example, Neville could be there, exploring the different plants and herbs that are associated with Beltane. Luna Lovegood might also make an appearance, with her unique perspective on the magical aspects of the festival. And perhaps some local wizards and witches from the area where the Beltane celebration is taking place.
Dumbledore might also play a role. He has vast knowledge and experience. He could offer some wise advice or even provide a special magical item or spell that is crucial for the unicorn's healing.
Hermione could be there. She's always around to support Harry and might be observing the ability test.
Well, Dumbledore would most likely be a significant character. He might be one of the few people who quickly understand the implications of Harry being a mutant. He could help Harry navigate through the challenges that come with his new identity. Additionally, there could be some mutant characters from the mutant universe who cross over into the Harry Potter world. For example, a character like Professor X could be an interesting addition, guiding Harry in understanding and using his mutant powers in a responsible way.
Maybe there's a human friend who is really curious about Harry's new situation. This friend could be constantly asking questions and trying to get involved in Harry's alien - related adventures, adding some comedic relief to the story.
In such a fanfiction, perhaps Harry uses his magic skills learned at Hogwarts. He might first find the injured unicorn in the Forbidden Forest. Then, using a spell like 'Episkey' or some advanced healing magic he's discovered. The unicorn could be injured by dark creatures or a trap set by poachers.
Hermione could be there. Maybe she tries to figure out a way to get Harry out of the mirror world. Another possible character is Dumbledore. He might have some knowledge about the mirror that could help Harry in this situation.