When your writing out the notes for your character one thing that your often told to do is to write their goals, what they want most.
In Sci-Fi you often have characters who do things like save planets, or stop invasions, and while you can use grandiose ideas like these, but I would recommend against it.
Most people have simple desires when you really get down to it, they may want a wife, to be safe, or to have kids, or the power to destroy their enemies.
Simple is better, even a simple goal can create an interesting character. If the planet is being invaded then your simple character, who only wants to protect his family, may have to leave and go on an adventure.
Also don’t be afraid to give your character contradictory goals, real people have those all the time.