Touma Kamijou also makes appearances at times. He has a special ability in his right hand that can nullify various powers, and his interactions with the characters from the 'A Certain Scientific Railgun' world add interesting dynamics to the story.
In this volume, the story focuses on the characters' efforts to solve a complex scientific mystery while dealing with powerful adversaries. There are also some unexpected plot twists that keep you on the edge of your seat.
Mikoto Misaka is definitely one of the most popular. She's the main character in the original work, and in fanfiction, people often like to expand on her story, her relationships, or even give her new powers or challenges.