Kanaruzawa Sekai, written 神鳴沢 セカイ in japanese, is, well, a God of the place where the story is set and a main character of it.

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As a god, she (she looks like a maiden) has some duties that she had to do, as it was revealed in the lastest chapters. She live alone within her mansion and work with some organizations that offer her "living sacrifices".

It is stated that she's not omniscient, nor omnipotent although she is very (very) old and already shown having some useful powers as a God. It is also stated by her maid (who also have some strange powers) that she is really incredible. As of manga chapter 33, it is revealed that she has the ability to foretell someone's past and future.

She was very shoked when Kirishima Yuuki proposed her when he was bringed at her house as a sacrifice. Before their encounter, she was feeling very lonely, even having some strange habits such as drinking alcohol and smoking. Before that event, she had almost never been outside her house, making her completly unaware of the human world. She slowly started  to show love feelings toward Yuuki.

Her duties as a deity are somewhat mysterious and appear to have a link with a kind of higher beings and with the world's genesis as well.