Since we don't have much information, the main character is surely the girl. There might be librarians as supporting characters, and perhaps other library patrons or friends of the girl could also be part of the story.
The girl is the central figure in this side story. Other possible main characters could be a wise old librarian who gives her advice or a fellow book - lover who becomes her friend. There could also be a character who causes some sort of conflict in the library, like a bully who tries to steal the girl's favorite books or a rival in a library competition.
Well, without more context, it's difficult to say precisely. It could potentially be a story that focuses on the girl's personal growth within the context of the Zell library. For example, she could be learning new things, facing challenges like dealing with strict librarians or competing with other students for limited resources, and all these experiences form the side story.
It's possible that there are other magical girls or boys in the side story too. They could be from different regions of the magical world, each with their own set of powers and personalities. Maybe there's a rival magical girl who at first seems like an enemy but later becomes an ally. And don't forget about the villains. There could be a powerful dark sorcerer or a group of mischievous imps causing trouble in the magical girl's side story world.
The main character is usually a male protagonist who embarks on the journey in the world of monster girls. There are also various monster girl characters, such as the lamia, harpy, and slime girl, who play important roles in the story, either as allies or antagonists depending on the situation.
I'm not sure specifically as it depends on the particular side story. But it could be some popular monster girl characters from the main Monster Girl Encyclopedia, like the lamia or the harpy.
Based on the title, the main characters are likely to be the succubus and the witch.
Obviously, from the title, a succubus and a witch are likely to be main characters.
Since it's about the 'girl's side', the main characters are likely to be female. They could be friends, sisters, or classmates. Without more details, it's impossible to name them specifically.
Tamamo is definitely one of the main characters. There might also be other characters that are related to her story, perhaps some other monster girls that she interacts with regularly. But without more in - depth knowledge, it's hard to say exactly who all the main characters are.
In 'west side side story', Tony is a key character. He is part of the Jets gang. Then there's Maria, a beautiful young woman who is Bernardo's sister. Bernardo is the head of the Sharks gang. And Riff, as the leader of the Jets, plays an important role in fueling the gang rivalry. Their actions and relationships drive the whole story forward.
I don't know exactly as there isn't enough information given. But I assume they are girls who are involved in the battles.