Dumbledore could also be a key figure. He has so much knowledge and wisdom that he might know some ancient magic or spell that could cure Harry. He could guide Harry through the process of getting his health fixed and offer moral support as well.
Well, perhaps Harry would finally be able to fully focus on his studies at Hogwarts without being held back by any lingering health issues. He could become even more proficient in magic, and might be able to take on more challenging tasks and adventures.
Harry is, of course, the main character. Neville could be an unexpected helper. Maybe he has come across a rare herb during his Herbology studies that has the potential to heal eye problems. And Luna Lovegood might also be part of it, with her unique insights and knowledge of the wizarding world's more esoteric aspects, she could offer some unorthodox but effective solutions for Harry's eye problem.
Obviously, Harry Potter himself is the central character as the story is about him getting top owl results. Hermione would also be important as she is always the one to help Harry with his studies.
Obviously, Harry himself would be involved. And of course, there would likely be healers at St. Mungo's Hospital for Magical Maladies and Injuries. Hermione might also be there to support Harry, using her vast knowledge to help understand any complex medical conditions or procedures.
Maybe Dumbledore, if he were still alive in the fanfiction. Dumbledore was highly respected and had a great influence. Knighting Harry would be a way for him to publicly recognize Harry's achievements and heroism on a grand scale.
The key characters could be Harry's muggle relatives. The Dursleys have been his legal guardians in the muggle world for a long time. They might be fighting to keep Harry in their home. On the wizarding side, Hagrid could be important. He was the one who introduced Harry to the wizarding world. He might step in to ensure Harry is placed in a proper wizarding family or environment. Also, Hermione and Ron could play a role. They are Harry's best friends and would surely fight for Harry to be in a place where he can be happy and learn magic freely.
Snape could also be a possibility. If Harry did something that Snape considered a serious offense in Potions class, like using an ingredient in the wrong way that could have caused an explosion, Snape might send a howler to scold him. However, it would probably also be filled with his usual sarcasm and dislike for Harry.
Harry is the central figure in need of help. Queen Elizabeth (assuming she is still part of the story's context) could be a key character. Her wisdom and authority within the royal family could be crucial in providing the kind of help Harry needs. Also, Meghan Markle could be involved. As Harry's wife, she might be part of the situation, either being in need of help along with Harry or playing a role in how the royal family's help is received and utilized.
It could be someone who has been writing fanfics for a long time and wanted to take a very bold step with this concept. They might be a part of some underground or niche fanfiction circles where such unconventional ideas are more common. However, without more information, it's almost impossible to know for sure who the author is.
Lupin could be a likely candidate. Given his experience and his role as a sort of mentor figure to Harry, he could give Harry the talk. It might involve Lupin sharing his own experiences with being a werewolf and how to deal with secrets and prejudices in the wizarding world.