Obviously, the Black Arrow himself is a main character. He is usually depicted as a skilled archer and a brave fighter.
There could also be antagonists. A cruel warlord or a devious sorcerer who wants to stop the Black Arrow from achieving his goals. These characters add conflict and drama to the fanfiction stories.
Not having read 'the black arrow graphic novel', it's difficult to name the main characters precisely. However, it's common for a graphic novel to have a central character with a unique personality. This character might be an outcast or a chosen one who is drawn to the black arrow. There could be supporting characters like a loyal friend who stands by the main character's side throughout their adventures with the black arrow, and an enemy who has a long - standing feud with the owner of the black arrow.
The main protagonist from the original 'the black arrow' is likely to be very popular in the fanfiction as well. Their heroic deeds and journey are often the focus.
The main characters are Green Arrow and Black Canary.
There could also be a villain. The villain would oppose the protagonist and create conflicts and challenges for the hero to overcome. Their actions would likely be the source of much of the drama in the novel.
It could be two star - crossed lovers from different social classes. One could be a noble and the other a commoner, and the love arrow brings them together against all odds.
The Arrow is sure to be a main character. In addition, Hawkeye would be a crucial character in this fanfiction. Since they both are archers, there could be a lot of interaction between them, like competing or collaborating in archery - related tasks. And of course, Captain America with his leadership skills would also be among the main characters to lead the team that now includes the Arrow.
Oliver Queen, of course. As the main character of Arrow, he is central to most crossovers. His skills as the Green Arrow and his complex personality make him a prime focus.
Well, often the main character is the one who gets the arrow to the knee. They could be a brave warrior or an adventurer. Maybe they have a unique name like Arlen or Lyra.
I don't know who the main characters are as I haven't read the 'day of the arrow novel'. It could be a brave warrior who is skilled with arrows. Maybe there's a wise old mentor figure who guides the protagonist in matters related to the arrow.
Without more details, it's hard to determine the main characters precisely. It could be two warriors from opposing sides, and an arrow fired in battle somehow leads to a connection between them that blossoms into love. Maybe one is a skilled archer and the other is a strategist who admires the archer's skills, and the arrow becomes a symbol of their relationship.