In some cases, villains from the original SVTFOE series could be main characters in relation to the evil star fanfiction. For example, Toffee. He might have some sort of connection to the evil star, either trying to use it for his own evil purposes or having some past history with it that is revealed in the fanfiction. There could also be new characters created specifically for the fanfiction that are related to the evil star, like a guardian who was tasked with keeping it in check but failed.
Obviously, Star is one of the main characters as it's about her Mewberty. Marco is also likely to be involved as he is her close friend in the series. He might play a role in helping her through the Mewberty process or just being there for her during the changes.
The Volturi are frequently in 'evil bella fanfiction'. Since Bella's actions, if evil, would likely draw their attention. They would be the ones trying to maintain order in the vampire world and would see Bella as a threat. Alice might also be involved as she can see the future and would be trying to predict Bella's next moves.
Naruto himself is often involved as the story usually revolves around the impact of the 'evil pieces' on the main characters. Sasuke could also be a central figure as his complex personality and his own search for power could make him a target or a wielder of the 'evil pieces'. Kakashi, being a strong and influential character, might also get involved in the events related to the 'evil pieces' to protect the village or his students.
Obviously Naruto himself is a main character. He's usually at the center of the story as he's the one dealing with the influence of the Evil God. Sasuke is also often involved, maybe as a rival or an ally depending on the plot. Sakura might be there too, trying to help Naruto or deal with the chaos the Evil God brings.
Obviously, Fox McCloud is often one of the main characters in such fanfiction. Since he is the leader of the Star Fox team, his capture would be a big deal. Falco Lombardi might also be involved, either being captured along with Fox or being part of the rescue team. Peppy Hare could be there too, perhaps using his wisdom to figure out a way to free the captured members.
Alina is also frequently involved. Her connection to the Darkling in the original work makes her a natural addition in many fanfictions. Their complex relationship can be further explored or even rewritten in different ways in fanfiction.
In many Mega Man Star Network Wily fanfictions, Proto Man can also be a major character. He has his own complex relationship with both Mega Man and Dr. Wily. Sometimes he might be trying to stop Wily on his own terms, or he could be conflicted about which side to take. Additionally, some fan - created characters might also be introduced to add more depth to the story, like a new robot created by Dr. Wily that has unique abilities and a personality that challenges Mega Man in different ways.
Star and Marco are usually main characters. Star with her magical powers is often at the center of the action during Festivia. Marco, as her friend, is right there with her. They face various challenges related to the forces of evil during this time. Also, characters like Tom might make an appearance as he has his own relationship with Star and could add some interesting drama to the Festivia - themed fanfiction.
The main characters in the love story are Star and Marco.
Definitely Star. Her 'paws on hips' pose is a focal point. Tom might also be a main character in these fanfictions. He has his own complex relationship with Star. In the fanfic, when Star is in her 'paws on hips' stance, it could be in a situation where she is dealing with her feelings for Tom, or they could be working together to face a common threat.