In literature and film, a protagonist, main character, or lead character was the character the story was based around.
In 2153, T'Pol found the protagonist of the movie Frankenstein interesting. When Trip Tucker assumed she meant Dr. Henry Frankenstein, but she corrected him, saying she was referring to Frankenstein's monster instead. (ENT: "Horizon")
In an anti-time future, Geordi La Forge was a published author. Jean-Luc Picard mentioned that he had read La Forge's last novel, but found the protagonist "a little too flamboyant." (TNG: "All Good Things...")
The holonovel Photons Be Free frequently described the character of The Doctor as "our protagonist" in the individual chapter's subtitles. (VOY: "Author, Author")
In 2372, Jake Sisko described his autobiographical novel idea to Onaya. One of the plot points was that the main character's mother died, though that wasn't the main thrust of the story. (DS9: "The Muse")
In 2373, Jake was working on a crime novel and asked Quark for advice on how to write nefarious characters. He felt that if he could observe Quark in action, it would give him valuable insight. Jake even offered to model the story's lead character after the Ferengi. (DS9: "The Sound of Her Voice")