The protagonist is a hero or a would-be-hero, someone who has decided to save the day or otherwise jump into action to do something about the problem, unlike the rest of the populace who cover in terror in the face of danger and oppression.
The first video game about Hero protagonist was released in 1983.
* Characters following orders are not eligible. These are usually minions (soldiers), servants or guns for hire and such who do it because it's their job to do it.
* Characters out for revenge, no matter how much the guy they intend to kill is the Evil Overlord of the Known World and Bringer of Doom, are not eligible - they do it for personal reasons, not because they want to save everyone else (as in, there's no sense of heroism involved).
* Characters who are driven by events outside their control towards a lofty goal that will eventually make them a hero of sorts are not eligible.
and so forth...
* Less straightforward cases may be tough to judge, Heretic II for example. Corvus is a hero, but only because of what he did in Heretic (1), not how the story unfolds in Heretic II and therefore would only be suitable as border case (but I remember little of the story, so it may be that it turned into a hero tale at around the time he speaks with the queen).
* There may be cases where the character wants to become a hero and is therefore eligible for this tag but in the end does more harm than good and is treated as a menace on par or worse than the thing he/she went out to oppose (he would still be a her as far as this tag is concerned as that's what the protagonist is trying to be).
There's also the selfish heroes who want to become heroes only to gain fame, admiration, wealth and such and not for selfless reasons one would expect from someone labeled as hero. These do belong to this tag despite it.