Creating a good character in any story is largely similar but there are special considerations which need to be examined when you are writing for science fiction. You must take into account the reader of science fiction, who are likely to prefer a specific type of character. You should also consider the differences between the characters world and ours and how that would effect their personality. This will vary far more based on the type of science fictional universe the character is in but if you do not take it into account it will likely feel false to the reader.
How Your Science Fiction Character Relates to Technology
The first thing to consider when creating a science fiction character is the science you want to use in the story and how he or she relates to it. This is important for a number of reasons. The most basic is that if you can use a character who does not understand the technology you can explain it to them without it feeling as much like exposition. On the other hand, if your character is a scientist who discovers the technology it is easier to explain why only he or she has it. These are both worthwhile and when creating a good science fiction character the key is to know how you want the character to relate with the technology.
How Your Science Fiction Character is Effected by Environment
The second thing to consider is how the world differs from ours. If you have a character in a utopian world he is likely to have a childhood very different from one now, and certainly different than one who grew up in a dystopian world. This becomes a bit less important in a world where it is similar to ours, but even then it can effect them. After all, kids who grew up now have a very different relationship with technology than those who grew up twenty years ago even though in many basic ways the world has not changed all that much, so imagine how much different they will be as technology advances.
Alien Science Fiction Characters
The other type of character you are able to create in a science fictional universe is the alien. This type of character gives the science fiction writer a lot of opportunities and they come in two types. The first is the humanoid alien, these generally are similar to human but allow you to explore specific parts of the human experience. Creating these characters often makes your science fiction feel a bit less realistic but realism is not always the point.
The other type of aliens are true aliens. This comes down largely to motivation. They need to want something. There may be other motivations but as long as they have this they can work as a character, even if no one really knows what that motivation is besides you.
The science fiction character, in the end, is still a character and usually human, but it is still worth your time to think through how the changes in the world would change that character and how it can reveal the ideas you have.