Characters, needless to say, play an important role in any story, let alone a science-fiction story.
There are three types of characters in a typical science-fiction story: the protagonist, the antagonist, and the minor/sacrificial characters. The protagonist is always the hero in science-fiction tales. He/she can be a normal human, a supernatural being, or a combination of both: a human with supernatural powers. Along with this, he/she is always successful in defeating the evil antagonist. The antagonist is usually a supernatural being of some sort (ghosts, aliens, etc.). They also must have one or more weaknesses enabling success for the protagonist. The antagonist may be evil from the beginning of the story, or could have turned to the "dark side". An example of this would be the movie series Star Wars. Minor characters in a story are usually in the protagonist's party. These characters have somewhat significance to the protagonist, and may even be close friends or family. During conflicts with the antagonist(s), minor characters may die in order to forward the
plot.
The plot(s) of a science-fiction story are similar in a sense that there is always a type of war or an important conflict. Some popular conflicts are: Good vs. Evil, Human vs. Unknown, and Natural vs. Supernatural. Usually, the plot progresses with a significant event. The protagonist and the antagonist often begin at the same level, and both train themselves or gain special abilities. With the death of a minor character, the antagonist becomes more powerful, which motivates the protagonist to strive for survival. The protagonist usually rises to the top, and always wins the battle he/she is in. In a non-series story, a science-fiction plot must have victory for the protagonist.