The main characters are Goku Black (Zamasu in Goku's body), Goku, Vegeta, and Zamasu in his original form. Goku and Vegeta are the ones trying to stop Goku Black's evil plans.
In the Goku Black story, of course Goku Black himself is a central character. He is really Zamasu who has taken over Goku's form. Then there's Goku, the original owner of the body that Zamasu stole. Vegeta also plays a major role as he teams up with Goku to fight against Goku Black. And we can't forget about Zamasu in his true form. He is the mastermind behind Goku Black and his actions are the root cause of the whole chaos.
Goku Black is a major character. He brings his Saiyan strength and his twisted sense of justice from his own timeline. Android 21, on the other hand, is a character full of mystery. In fanfiction, she often has her own motives and goals that interact with Goku Black's, whether it's to gain more power or to find a place in the world. Other characters might also be involved, like some of the Z - Fighters who could be either allies or enemies in the story depending on the plot.
There could also be other Z - fighters who play supporting roles. They might come to Bulma's aid when they realize that Goku Black has some sort of interest in her. Their presence could add more action and drama to the fanfiction.
Zamasu is often paired with Goku Black. Since they are closely related in the original story, fanfiction writers like to explore their relationship further. It could be about their shared goals or how they interact on a more personal level.
Future Mai could also be a key character in Goku Black forced fanfiction. Given her relationship with Trunks and her own skills, she could be part of the group that is fighting against Goku Black, or she might even have some unique interactions with him that drive the plot forward.
Well, obviously Black Goku himself is a main character. He is the center of the story in most cases as his actions drive the plot. Goku also features prominently as he is the one who has to oppose Black Goku. Vegeta is often there too, as he is another strong Saiyan and is eager to take on the threat that Black Goku poses.
Another possible main character could be the villain that forces the two Gokus to meet. This villain could be a new, super - powerful threat that the two Gokus need to deal with. Their meeting becomes a crucial part of the story to defeat this powerful foe.
It could be Zamasu. Since Goku Black was Zamasu in Goku's body in the original story, perhaps a more powerful and enraged Zamasu could be the main antagonist. He might be jealous of the new fused form's power and seek to destroy it.
Obviously, Goku and Goku Jr are the main characters.
There could also be other characters from the Dragon Ball universe making an appearance. For example, Vegeta might show up. He could be curious about Fem Goku and how she compares to Goku in terms of power. Or Bulma could be there, perhaps she's interested in the technology or biology behind Fem Goku's existence.
Well, obviously Goku Black himself is very popular. Writers often focus on his character, his power, and his motives.