As I haven't read the book, I can't be certain. But Amy herself is surely an important character. There might also be her family members, like her parents or siblings, and perhaps friends or romantic interests.
Not having read 'Amy's Story A Novel', we can only make assumptions. Amy is the central character, of course. There may be a best friend who is always there for her through thick and thin. Maybe there's an employer or a teacher who has a significant impact on her development. And it's possible that there are rivals in different areas of her life, such as in a competition for a job or a romantic relationship, which would make them important characters in the story as well.
Definitely the horses! In Heartland, the horses are like family. They could play a role in the fanfiction, like being part of the wedding procession or having some special connection to the couple's relationship. Also, Lou, Amy's sister, could be important as she might be involved in the wedding preparations or giving advice to Amy.
Obviously, Amy is an important character as she is the one who gets sick. There might also be her best friend, who could play a crucial role in taking care of her and being by her side throughout her illness.
Barry Allen, who is the Flash himself, is very important. Also, Iris West.
Oliver himself is surely an important character. After all, it's his story.
There might be a young protagonist, perhaps a child who is excited about Christmas. This character could be central to the story as they experience all the Christmas - related events. There could also be a parent figure, like a mother or father, who is responsible for creating the Christmas magic at home, buying gifts and preparing the dinner.
Beowulf himself is of course the most important. Then there's Grendel, the monster that terrorizes the Danes. Grendel's mother is also important as she seeks revenge for her son's death. And Hrothgar, the king of the Danes who Beowulf helps.
Another important aspect is her influence on public perception of health issues. She worked on communication strategies to make the public aware of the importance of following health guidelines. Through her work, she was able to increase public understanding and cooperation in matters related to public health.
As we don't know the full story of Shadow and Amy's family, it's hard to say for sure. But generally, important events could include a family member's birth, a big move to a new place, or an event that tested their family bond like an illness or a financial crisis.
Since I don't know the 'amy love story', I can't say who the main characters are. It could be Amy herself and someone she has a romantic relationship with, but that's just speculation.
Amy Fisher is obviously one of the main characters. And there are others involved in her story as well, like Joey Buttafuoco.