A thirteenth apostate is a sad former follower of a god. The apostate was usually used to go get coffee and donuts, whereas the other twelve got all the miracles and press. The apostate got tired of all the thankless abuse and ditched the god. It may be noted, however, that the perceived mistreatment usually steams from some misconduct on the part of the apostate.
Thirteenth apostates have a deep hate for all faithful followers of gods, and seek to kill them at every possible turn. They can be nasty, but are often killed by pets.