As a child, Gaku Yashiro was obsessed by things like psychology and people's reactions to specific situations. As he grew older, Yashiro became bored with his everyday life, where nothing interesting ever happened.One day, Yashiro's abusive older brother convinced him to lure a young girl to a shed, at which point Yashiro stood guard as his brother raped the girl. Yashiro then used kind words and presents to manipulate the girl into remaining silent about the incident.Yashiro and his brother made the process a monthly occurence, where Yashiro's brother would get to rape young girls monthly, while Yashiro himself would get the "thrill" he wanted out of his life by getting the chance to manipulate the girls, which also appealed to his intrigue with psychology.One day, Yashiro was approached by one of his classmates who had accidentally come into possession of a box of baby hamsters. Yashiro told her he would deal with the hamsters for her, at which point he went home, dumped them into a jar of water, and left them to drown.Returning hours later, Yashiro was thrilled to find one of them had survived, decided to keep it as a daily reminder of the incident and the thrill he experienced, and named his new pet "Spice".During one of Yashiro's brother's monthly rapes, due to Yashiro's own negligence, the boys' mother came very close to the shed where the rape was happening, and Yashiro's brother, attempting to keep his current victim from screaming, accidentally strangled her to death.Yashiro and his brother tried to revive her, to no avail, so they hid the body, though Yashiro quickly realized his brother planned to frame him for the murder.At some point, Yashiro's hamster died, and while Yashiro felt a sense of loss, it wasn't out of care for his pet. It was because he now had nothing to entertain him in his day-to-day life.Refusing to be framed by his brother, Yashiro planned to murder his sibling, then make it look like the boy killed himself out of guilt for raping and murdering the young girl from before.Succeeding in his plan, Yashiro found the new "thrill" for his daily life: Getting away with murder and manipulating innocent people.