Nagumo has a doppelganger older sister, Guest. Guest was a famous elite killer prospect at JCC, a genius talent ranked at the top of the Intelligence, Poisoning, and Assassination departments. When Guest graduated from JCC, her younger brother Nagumo Yoichi enrolled. Time passed quickly, and Guest joined ORDER, active as one of the strongest killers. Later, Nagumo and Sakamoto joined her, and the three of them became known as the strongest trio of killers, spending their daily lives together.