It might be an old hermit. He has lived in seclusion for a long time and knows about this secret thing. The story could be about how others try to find him and learn about the secret from him, or how he decides to share or protect this secret thing that he has guarded for so long.
It could be a young detective. Given that it's a novel about a secret thing, a detective would be a likely main character as they are often in the business of uncovering secrets.
It's hard to say precisely, but it's likely there are characters with different motives related to the secrets. There could be a character who uses the secrets for their own gain, like blackmail or manipulation. And on the other hand, there could be a character who is fighting against the secrecy, trying to bring the truth out for the sake of justice or just to clear the air.
Given that we have no details about 'every secret thing story', we can only guess. They might be people from different walks of life, such as a young person trying to keep a family secret, an older character who has held onto a secret for a long time, or a group of friends where one of them has a secret that could change their relationship dynamic.
Well, it's most likely the wizard. Since it's called 'a wizard's secret novel', the wizard would be in the center of the story, dealing with whatever secrets are involved.
The central theme could be mystery. Since it's a 'secret thing', there might be a lot of elements that are not immediately clear, and the story could revolve around uncovering that mystery.
The main character in a secret admirer story is often an ordinary person with extraordinary feelings. It could be a person who has noticed someone special in their daily life, like at the local coffee shop. They might be an introvert who finds it difficult to approach the object of their affection directly. So instead, they take on the role of a secret admirer. This character is driven by their emotions but held back by their insecurities, which makes for an interesting dynamic in the story.
I'm not sure specifically who the main characters are without reading the novel, but they are probably the ones driving the plot forward. They could be the ones with the most significant secrets or the ones trying to uncover them.
One of the main themes could be secrecy itself. The characters might be hiding things that gradually come to light, creating tension. Another theme could be guilt. If they are keeping secrets, they might feel guilty about it, which can drive their actions and relationships in the story.
It could be Bilbo. Since he already had so many adventures in the original story, there could be a secret side of him that the fanfiction explores. Maybe he has a secret mission that he never told anyone about, not even Gandalf.
It could be an original character. Maybe a young ninja from a small, forgotten clan who discovers they have the Mangekyo Sharingan. This character would have to learn about their power, its limits, and how to use it in a world full of powerful shinobi.
There's also a chance that the main character could be a completely new character from a minor clan in the Naruto universe. This character could have some hidden bloodline limit that makes them a prodigy in the ANBU. They might start as an underdog but quickly rise through the ranks due to their exceptional skills and intelligence.