Non-human Characters in Science Fiction
It's a cliché that sci-fi fiction and film are full of aliens. There's a reason that cliché exists—it's because it's true. Aliens are among the non-human characters that we'll find in sci-fi. But sci-fi isn't just about aliens. It's about robots, for example, or people who are just a little bit more (or less!) than human, like Frankenstein'smonster.
Sci-fi, in other words, is very much about exploring the limits of being human. What exists beyond us regular folk living on earth? Suppose there are "extraterrestrial" creatures. Would they be like us or would they be different from us? And what about machines? Can't they have feelings, too? By focusing not only on human, but non-human characters, sci-fi writers force us to consider what we even mean by the "human."
Chew On This
The monster in Mary Shelley's Frankenstein is not only one of the most famous characters in sci-fi: he's one of the most famous characters in all of literature. The thing is, we can't tell whether he is or isn't human.
Who knew that Martians have romantic relationships just like we do? Well, Ray Bradbury's Martians in The Martian Chronicles do. Delve into the ins and outs of the relationship of a Martian couple here.