This category is for atheists, characters who do not believe in a god (or gods). They may believe in beings that call themselves gods, such as Zeus or Hera, but simply perceive them as powerful beings rather than actual gods.
Its placement in "Characters by Religion" may be technically incorrect because atheism isn't a religion, but it's the closest thing. It is organization "by lack of religion" in that they do not ascribe to a religion that involves believing in or worshipping a god or gods. It should be noted that there are some atheist religions which involve supernatural belief systems that do not involve deities.