Kero and Kero Sei are twin brothers. They are no different from any other twins, except that when they came into this world, no one had any expectations for them. Because they were the children of their mother's affair with her husband's younger brother. Their father's younger brother - who was also their biological father - died in a racing accident. The only impression Zero and Sei had of their mother during their childhood was of a woman lying in bed, with a body temperature as cold as ice and smelling only of medicine. And their mother repeatedly reminded them: In this world, the only person Zero can rely on is Sei, and the only person Sei can rely on is Zero.