There could also be characters like Kakashi who might notice Ino's behavior towards Naruto. His role could be more of an observer, but his presence can add an element of authority and guidance. But again, the main relationship that drives the fanfiction is between Ino and Naruto.
Daphne could be a shy and artistic girl. She might be into painting or writing, and she sees Harry as this source of inspiration. Maybe she first noticed Harry in an art gallery or a library, and that's when the crush started.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
The main characters are surely Astoria and Harry. However, in fanfiction, it's possible to introduce new characters related to Astoria's family or background. For example, her parents might have certain expectations for her that conflict with her feelings for Harry. Or there could be an older sibling who gives her advice on how to handle her crush. And of course, characters from Harry's world like Dumbledore or Snape could make appearances in the background, influencing the overall atmosphere of the story, even if they're not directly involved in the Astoria - Harry relationship.