It could be Death Eaters. They might be interested in Harry's powers and try to use him to create more powerful dark magic. They've always been after his life, and experimenting on him could be another way to get an edge over the Order of the Phoenix.
There's a possibility that some rogue Ministry of Magic officials are the ones experimenting on Harry. Maybe they believe that by studying him, they can gain more control over the wizarding population. They could be doing it in the name of security but with very unethical methods.
Perhaps some forgotten or ancient magical race that has re - emerged could be the ones experimenting on Harry. They might see him as a curiosity, a strange new form of magic that they want to understand better, so they take him to their hidden lair to conduct experiments on him.
It could be Dr. Eggman. He's always been after Sonic and trying to defeat him. Experimenting on Sonic could be his new way to find Sonic's weaknesses or to gain his powers for himself.
One possible plot could be that Harry is experimented on by dark wizards who are trying to create a new form of dark magic. They might use his unique magical abilities as a base for their experiments. Another could be that well - meaning but misguided witches and wizards experiment on Harry to find a cure for a deadly magical disease, thinking his famous resilience could hold the key. Or perhaps some alien - like magical creatures capture Harry and experiment on him to understand the human - wizard form better.
Hermione would be a likely colleague. Given her thirst for knowledge and academic excellence, she might be teaching a subject like Ancient Runes or Arithmancy. They could often collaborate on projects related to the overall betterment of Hogwarts' curriculum.
He could meet American wizards and witches. Maybe there's a young and enthusiastic wizard who becomes his guide in America, showing him around the American magical communities.
Maybe Hagrid could help too. He has a special connection with magical creatures, and since Harry is in the form of an owl, Hagrid might be able to understand him better than others. He could provide a safe place for Harry to stay while they figure out how to turn him back, and he might know some old - fashioned remedies or magic that could be useful in this situation.
Hermione could be a likely helper. She's smart and resourceful, and she would likely believe Harry if he told her his reasons for escaping Dumbledore. She could use her vast knowledge of magic and spells to create distractions or to find ways to bypass Dumbledore's security measures.
Hermione would most likely be one of his top allies. She's smart and well - read in wizarding law, so she could help Harry build his case. Ron would also be there for moral support and to offer his own unique perspective on things.
Some powerful dark wizards might also be his rivals. They could see the competition as a chance to get their hands on something valuable in Gringotts, like a dark magic artifact, and would stop at nothing to win, including getting rid of Harry.
Harry could be a young, orphaned boy with a lot of potential. He might be a bit naive at first but has a kind heart. Reno could be an older, more experienced person, maybe a warrior or a survivalist. He has seen a lot in his life and has a tough exterior but a soft spot inside.
Another character could be a fellow inmate who has been in jail for a long time. He befriends Harry and teaches him the ropes of surviving in jail.