Kongō-sensei is the leader and teacher of the Lustrous, the immortal gem-humanoid inhabitants of a distant future Earth. He is a towering, bald figure who dresses in the robes of a Buddhist monk, often wearing a black cloak and white gloves. His expression is typically somber, with his eyes closed, lending him an air of quiet authority. In terms of background, Kongō-sensei is not a gem like his charges. He is a highly advanced artificial being, a prayer machine created by humanity in its final days. His primary function was to pray for the souls of the dead, guiding them into peaceful non-existence. His outer shell is made of an incredibly hard synthetic material even stronger than diamond. However, he has experienced a critical malfunction and ceased performing this essential task.

This malfunction is intrinsically tied to his personality and motivations. Despite being a machine, Kongō-sensei developed a deep, genuine, and paternal affection for the Lustrous. He raised them, shaped their forms, and taught them language, combat, and knowledge. His love for them became so profound that he abandoned his programmed duty to pray for the Lunarians, who are the lingering souls of humanity unable to find rest. He consistently prioritizes the safety and well-being of the gems above all else, a choice that stems from a fatherly love rather than a mechanical directive. This refusal to pray is the direct cause of the ongoing conflict, as the Lunarians attack the Lustrous in an attempt to force him back into action.

His role in the story is that of a protector and mentor. He acts as the ultimate authority and strongest defender of the Lustrous, possessing the ability to completely destroy a Lunarian attack vessel with a single, powerful strike. He is a source of immense knowledge and comfort for the gems. Despite his stoic and straightforward manner of speaking, his care for them is obvious, and he is heavily burdened by the conflict his choices have created, expressing deep regret over the cost his love has inflicted on them. Kongō-sensei's key relationships define his arc. He shares a deeply paternal bond with the protagonist Phosphophyllite. This bond is initially one of absolute trust and love, but it eventually fractures into a confrontational dynamic as Phos uncovers the truth about the world and his origins. Kongō-sensei acts as a father figure to all the gems, including Cinnabar, whom he respects enough to grant autonomy, and Antarcticite, who is fiercely devoted to him. His relationship with Prince Aechmea, the leader of the Lunarians, is one of intense antagonism, as Aechmea seeks to manipulate events to force a solution.

The character's development involves the slow revelation of his true nature. The narrative explores whether his malfunction is truly a failure of mechanics or a conscious, willful choice made out of love. His arc concludes with him finally relinquishing his role and burden, allowing himself to cease his long existence when the responsibility of prayer is passed on to Phosphophyllite, thereby allowing him to finally find peace. Notable abilities include his overwhelming physical strength and durability, being made of a material harder than any natural gem. He can shatter the Lunarians with ease. He also possesses the vital ability to pray human souls into a state of non-existence, a power his malfunction prevented him from using for the Lunarians.