Not having read the novel, it's difficult to determine the exact main characters. However, based on the title, the princess is surely one of the main characters. There could be a love interest who might either support or be a hindrance to her secret fan life. Also, there could be a villainous character, perhaps a courtier who is against any deviation from the traditional royal image and tries to expose the princess's secret fandom.
Well, without having read 'the closet fan princess novel', it's hard to say exactly. But if I had to guess, it might revolve around a princess who is a fan in a rather unique or restricted situation. For example, she could be a fan of a particular art form that is not approved in her kingdom, so she has to keep her fandom a secret, like in a closet. This could lead to all sorts of interesting plot developments, such as her having to sneak around to pursue her interests or facing conflicts with the royal court over her hidden passion.
The unknown. You don't know what's inside the closet. It could be a monster, a ghost or something equally terrifying. It plays on our fear of the unseen.
Most likely not. 'Girl in the Closet' is probably a work of fiction. Stories like this are often made up to engage and captivate the audience rather than being based on true happenings.