Protagonist
The character whose quest and transformation carry the story — not necessarily a hero, nor necessarily likable.
The protagonist is the story's central character: the one whose desire sets the story in motion, whose choices make it fork, and whose transformation (or failure to transform) gives it meaning. Not necessarily a moral "hero" — Macbeth, Emma Bovary and Walter White are protagonists — nor even likable: they only need to be *interesting* to follow.
The operational criterion is agency: a protagonist acts, chooses, pays the price of those choices. The passive protagonist, a mere witness tossed around by the plot, is one of the most frequently flagged weaknesses in first novels — every key scene should pivot on one of their decisions.
Example
Emma Bovary: neither heroic nor exemplary, but every fork in the novel springs from her desires.