Eiko Teruya is a character who appears as a significant antagonist. A seventeen-year-old human, she is the daughter of the head of the influential Teruya clan, a family of Koga ninjas that holds a rivalry with the Hattori family. Her affiliation leads her to both serve as an agent of the Imperial Public Safety Commission and to enroll as a transfer student at Constant Magic Academy.

Eiko has green hair which she typically wears in a ponytail with side locks, and her eyes are also green. She wears the standard Constant Magic Academy uniform but is later seen in a ninja outfit during the Capital War arc. Her personality is one of her most defining traits, as she is mischievous, provocative, and openly manipulative. She is described as perverted and clingy, stating that once she becomes attached to something, she cannot leave it alone. This is most evident in her relentless pursuit of the protagonist, Akuto Sai. Her interest in him seems to begin as a mission or a form of teasing, but she soon becomes genuinely attached, finding him "cute" and clinging to him at every opportunity, often jumping onto his back unannounced to surprise him. She uses her sexuality as a deliberate tool, attempting to seduce Akuto to sway him to her family's side, though he remains largely unfazed by her advances.

Eiko's motivations are deeply tied to her family and her own ambition for power. While she claims she intends to simply tease Akuto, her actions reveal a more calculating and ruthless nature. She seeks to gain control of the Teruya clan and usurp the position of High Priest. To achieve this, she conspires to kill her own father, Keizo Teruya, and several of his subordinates, framing Akuto for the murder. She uses the chaos surrounding the appearance of the Demon King as a cover to seize command of the Koga ninja army, demonstrating a willingness to commit patricide and deceive others for her own gain.

In the story, her role evolves from a mischievous spy and nuisance to a primary antagonist. She infiltrates Constant Magic Academy to observe events and becomes fixated on Akuto. She also acts as a direct rival to Junko Hattori, who sees Eiko as an enemy and is alarmed when Eiko gets close to Akuto. The two eventually come to blows, with Junko confronting Eiko over her deception. Her scheme ultimately unravels when Junko, wielding a treasured sword, witnesses Eiko murdering her father. This truth is exposed by the student council president, leading to Eiko's arrest by the government.

Key relationships define her interactions. Her relationship with Junko Hattori is that of a rival from a competing ninja family, and the two share a deep-seated animosity. Her primary target is Akuto Sai, whom she both desires and sees as a powerful asset. For a time, she also works alongside Boichiro Yamato, a fellow agent, to carry out the assassination of her father and his men.

In terms of development, Eiko begins as a seemingly playful and mischievous transfer student, but she is gradually revealed to be a cunning and ambitious schemer who stops at nothing to achieve power. Her downfall is swift after her true actions are discovered, and she largely disappears from the narrative after being arrested. As a ninja, Eiko specializes in Ninjutsu. Her primary weapon is a tantō, or short blade. She is a master of stealth, able to hide her presence completely and ambush targets without warning. Her combat skills are considered comparable to Junko Hattori, who is ranked as one of the top fighters at the academy.