Kirishima Yuuki is the main protagonist of the story.
He has a dark past : he happily lived while his parents where running a pharmaceutical enterprise, when suddenly, some guys of an organisation who take care of Gods came into his house and said to his family (both his younger sister who had a brocon and his parents) that he will become a sacrifice for a god. And he came every days to repeat that.
The organization then ruined the enterprise of his parents, making them angry, and causing his strangulation by his very own father, while his sister was trying to stop him. He then accepted becoming that unknow god's sacrifice, who was none other god than Sekai.
He then encountered Sekai and when she asked him what he wanted in exchange for being a sacrifice, he merely answered that he wanted to "marry her". Afterwards, his wish was granted by Sekai and they start talking to each other, with Yuuki promising Sekai that he would make her happy.