Obviously, Harry would be one of the main characters. He is the one making the decision to break the alliance, so his character development would be central to the story.
There could be his friends like Hermione and Ron. They might play different roles. Hermione could be the one trying to talk Harry out of it at first because she sees the potential benefits of the alliance, while Ron might be more on Harry's side, understanding his gut feeling about the alliance.
The main characters in an 'emerald alliance harry fanfic' are hard to determine without reading it. However, it's likely to have a mix of original and new characters. There could be a character who is an outcast in the wizarding world but has unique knowledge about emeralds and becomes a key figure in the alliance. Harry could be a central figure if the story ties in closely with his adventures, or it could be a completely different set of characters who are driving the plot forward around the concept of the emerald alliance.
Well, Harry Potter himself could still be a main character. Since it's about family alliances, his friends' families like the Weasleys would likely be important too. Hermione's family might also be involved, even if they are Muggle - born, as they could contribute in unexpected ways. And who knows, some minor characters from the original series might step up and become main characters in the context of the family alliances.
One possible interesting plotline could be Harry's reasons for breaking the alliance. Maybe he discovers some dark secrets within the alliance that go against his values, like some members being involved in unethical magic practices.
It could be a group of young adventurers. For instance, a brave warrior, a clever mage, and a stealthy thief who form the core of the secret alliance.
Obviously, Harry Potter and Severus Snape are the main characters. Harry is the young wizard who is constantly under Snape's scrutiny at Hogwarts. Snape, on the other hand, is the complex and often - misunderstood professor.
Hermione Granger might also be a main character. Given her intelligence and knowledge of magic, she could be helping Harry in his quest to break magic. She might be researching ancient spells or looking for loopholes in the magical laws. Ron Weasley could be there too, providing support and his own brand of humor and loyalty as Harry undertakes this dangerous mission.
There might also be other characters involved, like Sakura. Since she was Tsunade's apprentice, she could be part of the story. Maybe she's caught in the middle when Tsunade has to take some action regarding Naruto. Also, Kakashi could be in the fanfic. He has a close relationship with Naruto and might be affected by whatever Tsunade does to Naruto or be involved in the events that lead to Tsunade's actions.
Gil - galad, the Elven king, is another main character. His elven wisdom and power were important in the fight against the enemy. His interactions with the human and dwarf leaders in the alliance would be a key part of the story, as they had to work together despite their differences.
Most likely, it would include the main characters from the Potter series like Harry Potter himself. Since it's about a broken alliance, his friends Ron Weasley and Hermione Granger would probably be involved as well. There could also be other important characters from Hogwarts or the wizarding world, such as Dumbledore or Snape, depending on the nature of the broken alliance.
I'm not sure specifically as it depends on the fanfic. But it could be the leaders of the alliance, perhaps a human general or an alien diplomat.