Protagonist
A protagonist is the main figure of a piece of literature or drama and has the main part or role. Alternatively, the phrase denotes a primary advocate of or proponent for a cause or movement. The main character can be a hero or a villain in a story - it is just the character with the lead role. In literature, the protagonist (Classical Greek πρωταγωνιστὴς "protagonistes") is characterized by his/her ability to change or evolve.
It should be pointed out that the protagonist is not always the hero of the story, as implied in the Grand Theft Auto games. They may commit a crime of some sort, but he/she is still considered the lead character, and is known to help out his/her allies or close associates.
The opposite of a protagonist is an antagonist, which is the main person working against the protagonist. Again, this can be a villain or a hero, though usually a villain.
Current list of GTA protagonists