Obviously, Harry would be one of the main characters as he is the one being captured. And then there would probably be characters from Abstergo, though we don't know exactly who without more context of the fanfic.
In this fanfic, Harry is the central figure as he's the captive. Abstergo agents would be major characters too. There could also be some of Harry's friends who might be trying to rescue him in the background, like Hermione or Ron, even if they're not directly involved in the capture part.
Well, in such a fanfic, Harry might be subjected to various interrogations by Abstergo. They could be trying to extract some sort of special knowledge or power from him.
Obviously, Harry Potter himself would be a main character. Elrond, who is raising Harry, would also be a central figure. There might also be other elves like Arwen, Elrond's daughter, who could play important roles. Maybe some of Harry's friends from Hogwarts could make appearances in the story, even if it's just in his memories.
I think Dobby could also be a significant character in this fanfic. Since he has connections to both the Malfoys and Harry. He might be the one who gives the Malfoys the idea to rescue Harry or helps in the rescue in some way. Also, Voldemort or his Death Eaters could be important in the background as the threat that Harry needs to be rescued from. Their presence would constantly loom over the story, making the Malfoys' actions more dangerous and significant.
Harry Potter himself could be a main character. He has a history of standing up against injustice, so it's likely he'd be involved in a revolt. Hermione would also be a key figure. Her intelligence and knowledge of the wizarding world's laws would be invaluable in planning and leading a revolt. Ron, with his loyalty and courage, would be right there with them.
Harry Potter would probably be one of the main characters. He's the central figure in the original series, so it's likely he'd be involved in this fanfic in some capacity, perhaps as the hitman or someone related to the hitman - style plot.
Harry is the central figure in 'harry turns into a snake fanfic'. In addition to his closest friends, there could be some new characters introduced. For instance, an old wizard who specializes in transfiguration gone wrong. This wizard could be a hermit living on the outskirts of the wizarding world. He might have the key to understanding Harry's transformation and reversing it. And we can't forget the snakes themselves. There could be a particularly intelligent snake that befriends Harry in his new form and helps him in his journey.
Depending on the plot of the 'harry gets even fanfic', some other characters from the Harry Potter universe might make an appearance. For instance, Professor Dumbledore, even though he's passed away in the original series. His wisdom might be a guiding force for Harry, either through memories or some kind of magical connection. Or characters like Luna Lovegood could also be involved, perhaps providing some unique perspective on Harry's need for revenge.
Obviously, Harry Potter himself is the central character. His journey in Slytherin would be the main focus.
There could be an antagonist in the fanfic. Maybe someone who had a vendetta against Harry Merchant and was somehow involved in his death. This character would add conflict to the story as the people left behind try to figure out what really happened to Harry.
Obviously, Harry would be the main character. He's the one with the unique heritage of being Hades' son. Then there would probably be Hades himself, who could play a big role in guiding or sometimes hindering Harry. Hermione and Ron would likely still be important characters as Harry's friends, though their relationship with Harry might be more complex due to his new identity. And there could be other demigods or underworldly beings like Persephone, who might have some influence on Harry's journey.