The String/3 Worlds

Between all worlds (heaven, the mortal realm, and hell) is a very, very long hole, and a string within in that connects all of these realms together. Periodically, denizens from all worlds will accidentally end up in the wrong world through this string; sometimes, those from hell will come up to the mortal realm, or those from the mortal realm will come up to heaven, or drop down to hell. 

Bijin is one of many guards that is tasked by God to protect the mortal realm from monsters from hell who are not willing to go back without a fight. 

The guards are not the only line of defense. Many settlements near these holes are asked by God to project a barrier around villages to protect the people from any monsters that do break through. Hime is the next priestess in line for the village where she and Bijin live.