I think that for most characters, writers use generic characters that do not require too much explanation. There are certain character models or archetypes that all of us are familiar with from stories, films and even real life. The advantage of using such characters in short stories is that it doesn't require much to get the reader to imagine and understand that character so it makes the writer's job a lot easier.
A common archetype is the "mother" archetype -- who deeply cares about her children and will do everything in her power for their happiness. There is also the "mentor" archetype, a wise role model and guide for other characters. And of course, the "villain" archetype, the evil character that always harms and competes with the other characters.