The main characters are, of course, the little men themselves. However, without more context, it's hard to say more about their individual identities. They could be scientists, explorers, or just ordinary beings caught up in extraordinary situations.
Well, since we only know it's about 'the little men science fiction', the main characters are the little men. They might have different personalities. For example, there could be a curious little man who is always eager to learn about new things in their science - fictional universe. There might also be a cautious one, who tries to keep the group safe from potential dangers. And perhaps a friendly little man who is good at making allies with other beings they meet during their adventures.
In 'the little men science fiction', the main characters are likely to be the little men. Maybe there's a leader among them, a brave little man who guides the group through various challenges. There could also be a character who is more of a thinker, coming up with plans and strategies for their survival or progress in the science - fictional world.