One of the main themes is self - discovery. The characters, like Hori and Miyamura, are constantly learning more about themselves as they interact with each other. Another theme is the acceptance of others' true selves. Hori discovers Miyamura's hidden side and accepts him, and vice versa. It also has a theme of friendship evolving into love. Their relationship starts as a friendship based on shared secrets and gradually turns into a romantic relationship.
Definitely Hori. She has to balance her school life, where she is popular and responsible, with her home life where she takes care of her younger brother. Her relationship with Miyamura also shows different aspects of her personality. And Miyamura, with his past that he has kept hidden from most people, is very interesting. As the story progresses, we see him open up more and more, which is a great character arc.