Simple Definition:

A protagonist is the main character in a story who faces challenges and drives [pushes or advances] the plot forward [the events that happen]. They are usually seen as the hero or the person the reader or viewer supports and wants to succeed. For example, in the story of “Harry Potter,” Harry is the protagonist because he is the main character who faces challenges and drives the story forward.

Very Simple Definition:

A protagonist is the main character in a story who faces problems and is the person the reader or viewer wants to win. For example, in the story of “Cinderella,” Cinderella is the protagonist because she is the main character who faces problems and we want her to have a happy ending.