I don't know as there's no information given. It could be a young girl who loves reading and spends her days on the beach with her books.
As we haven't been told anything about the 'beach reading story', the characters could be a writer who goes to the beach to get inspiration and reads other works while there. There could be a local librarian who holds a beach reading event and the participants of that event could be the characters. Or perhaps it's a story about a couple who reads romantic novels to each other on the beach at sunset, so the two of them would be the main characters.
Since we have no details, the characters could be diverse. There might be an old man who has a collection of sea - related books and reads them on the beach every day. There could also be a family who comes to the beach for a vacation and while the kids play, the parents relax with a good read. Another possibility is a group of students who are on a beach trip and use the time to study and read for an upcoming exam, so the characters would be those students.