Often, there is the phoenix itself, which can be depicted as a majestic and powerful being. There might also be a young hero or heroine who discovers the phoenix ashes and is thrust into an adventure.
Often, the main characters from the original Avatar series like Aang, Katara, and Zuko are still central in 'Ashes to Ashes' fanfiction. Their growth and new adventures are explored further.
I'm not entirely sure as it can vary depending on the specific fanfiction. But often, it could be the protagonist who has a strong connection to the past events that are being explored.
Harry Potter himself is often associated. Since he has a connection to the phoenix Fawkes in the original series, it's likely that in fanfiction involving a phoenix hybrid, he might play a role in discovering or befriending the hybrid. Hermione could also be involved as she is always curious about new magical phenomena and would surely be interested in a phoenix hybrid.
Obviously, Harry Potter himself is a main character. He is often at the center of the story, trying to understand the significance of the Black Phoenix. Hermione Granger is also commonly included. Her intelligence helps Harry figure out the mysteries surrounding the Black Phoenix, like researching ancient texts about it.
Harry Potter himself is often a main character. Since he has a connection to the Phoenix through Dumbledore's Phoenix, Fawkes. He might be the one to face the threat of the Dark Phoenix. Hermione Granger could also be a main character. Her intelligence would be crucial in trying to understand and combat the Dark Phoenix. Ron Weasley might be there too, providing his loyalty and friendship. Another character could be Draco Malfoy, perhaps in a situation where he is either an ally or an adversary in the face of the Dark Phoenix.
Obviously, Harry Potter himself is a main character in most of these fanfictions. He's usually the one who has some sort of encounter with the Ice Phoenix.
The main characters could vary greatly depending on the specific fanfiction. It might be an original character created by the author who is on a quest to rebuild a shattered world. For example, a young hero with a mysterious past that holds the key to rising above the ashes.
There might also be some secondary characters from the Marvel universe that play significant roles. For example, Nick Fury could be involved in guiding the Avengers as they try to rise again. Or characters like Doctor Strange might make an appearance to offer his magical expertise in helping the Avengers rebuild and face whatever threats are out there.
Often, there will be an antagonist or a group of antagonists as well. They could be those who oppose the rebuilding of the empire for their own reasons, such as wanting to keep the power structure as it is in the post - empire chaos or having a grudge against the old empire. These antagonists add conflict and drama to the story.
I don't know specifically as I haven't read this fanfiction. But if it's related to 'Star Wars Rebels' (assuming SWR stands for that), it could have characters like Ezra Bridger, Kanan Jarrus, Hera Syndulla etc. as main characters if the story follows the general themes of the show.