Well, clearly Light and Sayu are the main characters. They are the focus of the fanfiction and the story usually revolves around their relationship, actions, and interactions.
In the ending of the Higehiro light novel, the main characters' relationships reach a certain resolution. The story likely wraps up the various emotional arcs and developments that have been building throughout the narrative.
One characteristic is the focus on character relationships. In higehiro light novels, the relationships between the characters are often complex and evolving.