Maria Tengai is Shinichi Kuruma's deceased older sister. Recognized as the reincarnation of the Virgin Mary, she became a target for the group called the 13 Doctors or Apostles. They sought her unique blood to achieve their goal: reviving a godlike entity. She perished in Shinichi's arms during the "Great Destruction," catastrophic earthquakes that struck Shinjuku. This event deeply impacted Shinichi, fueling his motivations.

Maria's blood holds special properties essential for the complete resurrection of the omnipotent existence pursued by the antagonists. This connection extends to Ayaka Sanders, who received an infusion of stem cells derived from Shinichi. These "Almighty Cells" originated from the same source as Maria's blood. Consequently, Ayaka develops a psychic link with Shinichi and becomes targeted, as her body potentially holds the "blood of Maria" needed for the god's resurrection.

Maria's legacy shapes the central conflict. Her death drives Shinichi's quest for vengeance against his father, Dr. Genzou Kuruma, who conducted experiments on Shinichi and their mother. Furthermore, her blood remains the final component required by the antagonists. Dr. Kuruma later creates clones infused with isolated components of Maria's blood, including one named Noah, who plays a significant antagonistic role.