Every story has a hero... the main character who helps you discover the true potential of the tale you are telling. At the same time, though, the hero of your story isn't the only character that matters, at least he/she shouldn't be. The hero should be constructed as part of a network of characters, all inter-meshing with one another.
In comparing each character to your hero, you begin by defining them. This can be done in four ways... within the context of the story, by opposition to one another, the overlying archetype of each character, and by an underlying theme in the story itself. Every character should work toward advancing the goal of the story, or the premise. Each character has a part to play, otherwise they don't belong in the story. You'd be well advised to get rid of them.
The most important character in a story is the hero, the one who has found themselves in a predicament that drives the action. All other characters represent either an opposition to the hero, or an alliance... and sometimes, a combination of both. The hero makes a decision to go after a great desire but has a weakness that prevents them from achieving their goal.
An opponent blocks the hero from their desires, but more, they also desire the same goal as the hero. This forces the hero and the opponent to come into direct contact. The opponent doesn't necessarily have to be evil... in fact, the opponent might be a more moral person than the hero, even a friend or a lover. The opponent is just on the other side, that's all.
The hero might also have allies in a story... characters who help the hero move toward their desire and also help the reader to know the values and feelings of the hero. The ally may also have the same goal as the hero but sometimes they have their own goals and desires.
More layered stories make use of a combination of opponents and allies, such as a fake ally who is really an opponent in disguise. This character pretends to be the hero's friend but really has an agenda all their own. However, by acting as a friend to the hero, the fake ally often ends up inadvertently helping the hero achieve their desire.
Some stories make use of two or more main characters... a love story, for example, defines both characters equally... a person only becomes an authentic individual by entering into a relationship with another person on an equal footing. Still, even in love stories, one of the characters drives the desire line and so becomes the most powerful character in your story.
Another type of story with two main characters is a buddy story. These stories are more unequal but allow the author to examine the hero through the buddy, who is a sort of a mirror image of the hero. The buddy is both the opponent and the ally to the hero. One of the most intriguing parts of a buddy story is the conflict between the friends.
Each character in a story should be representative of a specific archetype, or psychological profile. This allows the author to create unique and authentic characters by comparing them to all the other characters in the story. This is done primarily through the theme, or the proper way to act, expressed in each of the characters.
Some questions to ask yourself: have you clarified the hero's goal in the story, and does it run the entire length? Have you detailed the opponents in ways that express the same desire as the hero?