The main character of a story or plot. Most forms of fiction have the protagonist as a hero but there are rare occasions with an evil or not-so-noble protagonist.
The most common form of protagonist is a hero who rises above the occasion to make the world a better place. They tend to be people who have experienced true pain but decided that they want to make sure nobody else will face suffering like that. Most forms tend to have them naive to the point of stupidity (until it is time to be serious then they become incredibly smart) but kind to everybody they meet.
Sometimes the protagonist can be a villainous person who is wicked in some perspective. When the audience gets a new view of the villain they may feel empathy towards the character or even cheer them on. With this new perspective we can understand why they are wicked and what the goal is. (Light Yagami for example)
The protagonist is meant to serve as a role model. In their position their good qualities are easily seen and the overcome their negitive aspects (even the villain too if they are protagonist).