Makoto and Kyoko get drunk during the former's 18 birthday party and they have a night of passion. Kyoko falls pregnant as a result, leaving the pair to make a big decision on their future as well as their unborn child's future.
Makoto was a normal 12-year-old boy until he kills his sister by accident. He meets Kyoko on the same night and the two become inseparable.
(This story is a AU, but the killing game still happens)