A hero is the firefighter who pulls you out of a burning car. The protagonist is the main character in the story you write about it.

A hero saves the day. Like a protagonist, a hero can be the main character, but it has to be the good guy. A hero is inspiring; just ask David Bowie or the B-52s. A heroine is the female version, but the language is changing to allow a woman to be a hero, too, as in this example:

She is the latest Star Wars hero and a hit with old and new generations of fans. (The Guardian)

Here's another example of hero:

She called him a hero and that if not for him, the 30 people inside the consulate would not have made it out. ( Washington Post)

A protagonist is the main character in a drama. Unlike a hero, a protagonist does not always do the right thing. A protagonist isn't any ol' character — you wouldn't have tenprotagonists, just one. A protagonist can also be the main character in a real life event. Examples:

We are all the protagonists of our own story, aren't we? (Time)

It's so gratifying that eight of the films on my list, many of which were mainstream hits, feature a woman or girl as the protagonist. (Slate)

The difference is in their Greek roots — the word hero meant "demigod" and a protagonist meant the first person to address the chorus in a Greek drama, also known as the main character.