Natsuki Seba is a young man in his early twenties, a student at the JCC and the head of its Weapon Production Department. He first appears working part-time at the Okutabi research laboratory, where he is an intelligent and capable inventor. He has a lean build, blue eyes with asymmetrical moles beneath them, and dark hair. His most notable creation is an optical camouflage suit that renders him invisible, making him a formidable combatant. Despite his talent, he often comes across as nonchalant and unbothered, disliking anything troublesome and even stating that friends are only useful for enjoyment. He is a pragmatic person who keeps a clear goal of saving a certain amount of money for retirement and insists on leaving work on time. However, beneath this cool exterior, he is fiercely protective of his younger brother, Mafuyu Seba. Their father was abusive and pushed both brothers into harsh assassin training, and Natsuki often took the blame and protected Mafuyu. To keep Mafuyu from being sent to a dangerous training facility, Natsuki volunteered to go in his place, showing his deep loyalty. Initially introduced as an antagonist, he battles Shin Asakura and Heisuke Mashimo during the lab siege. After the lab collapses, he temporarily leaves the assassin world and uses his inventions to run a successful crepe shop. Later, when his brother Mafuyu is kidnapped by the villain Slur, Natsuki re-emerges as an ally at the JCC. He helps Shin gain access to restricted data and provides him with weapons such as brainwave-blocking hoods and limiter-removing gloves. During the final clashes, he uses his invisibility suit to rescue Mafuyu and later aids Sakamoto and Shin in escaping. His development shows a transition from a detached hired killer to a reliable supporter who acts out of genuine care for his family and comrades. His inventive genius extends beyond stealth—he has built freezing bombs, electrified gloves, and other experimental devices, and his efforts were recognized with the Yotsumura Award for his graduation project.