Natsuki is a tall, slender, and bright young man, standing at 176 cm tall and weighing 67 kg. He is distinguished by his dark eyes and three asymmetrical moles (two on the right, one on the left), along with his characteristic medium-length, unruly, naturally messy dark blue hair. Natsuki is also a master weaponsmith from a young age, a top student in the Engineering/Mechanical Engineering department, and a research trainee at the JCC research institute. He uses a high-tech suit (Invisibility Suit) that allows him to become invisible to approach and attack his opponents. Natsuki always maintains a calm demeanor even in dangerous situations, often likes to strike a "cool" pose, and frequently keeps his hands in his pockets. He dislikes troublesome situations and often shows signs of weariness, yet he is a very caring person. Furthermore, Natsuki always loves and protects his younger brother, Mafuyu Seba (a boy around 14-15 years old who is also very clean). Willing to do anything—even risk her life—to prevent her younger brother from being sent to Al-Kamar (a terrifying orphanage).