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.
There's a chance that the main character is an outsider who gets involved with the wizard's secret. For example, a regular person from a non - magical world who accidentally discovers the wizard and the secret. This could lead to a story about how this outsider copes with the magical world and the secret they've found out.
Magic and mystery are likely to be main themes. Wizards are often associated with magic, and a'secret' implies mystery.
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 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 a young apprentice wizard who has just discovered his powers. He is eager to prove himself and protect the North from all the threats it faces.
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.
Perhaps it's an older neighbor. Someone who has more life experience and has seen the soccer mom grow up in the neighborhood. This neighbor could provide a different perspective on her secret desires and offer wisdom based on their own life journey.
I don't know as I haven't read it. It could be the baker, of course.
Well, one major spoiler is that the wizard discovers a hidden magical artifact that changes the course of his destiny.
Twilight Sparkle could be a great main character. She is very knowledgeable about magic in the MLP world. Facing a dark wizard would test her skills and knowledge to the limit, and she could learn a lot from this encounter.
Since I haven't read the novel, I don't know who the main character is. It could be the storyteller themselves, as the title seems to center around them.