I am probably being a little bit nitpicky with these questions, so please bear with me. When the assignment says "characters must be post-apocalyptic themed" and "avoid cliches", did your teacher provide an example? To me, the concept of a "themed" character runs parallel to cliches. For example, a post-apocalyptic archetype is the desensitized, violent youth, often a child born or raised during the apocalypse who is now much more violent and callous than its parents or even some of its friends. That, while fitting with the setting, is also common enough now that it is a cliche. So, while I realize that the assignment is calling for creative characters, I find those two rules together are somewhat useless and I'd like to know if there was clarification.

Also, do you have a world in mind? Zombies? Alien invasion? Nuclear war? Ruined city or jungle? Right after the apocalypse or centuries afterwards? Any inklings you have already will help us better assist you with your narrative.