Mikihiko Tsukimura is Mayu Tsukimura's older brother and an incubus. Deeply concerned for Mayu's well-being due to her severe androphobia and uncontrollable succubus powers, he orchestrates her relocation to live with Shungo Ninomiya, leveraging Shungo's apparent resistance. His long-standing friendship with Ryoko Ninomiya, forged through collaborating on high-risk activities like mercenary work and infiltrating conflict zones, facilitates this arrangement. To monitor Mayu's progress and ensure her safety, Mikihiko moves into the Ninomiya household alongside her, simultaneously taking employment as the school janitor.

His fierce protection surfaces through subtle, veiled threats directed at Shungo, implying severe consequences should Mayu come to harm. Beyond his guardian role, Mikihiko skillfully manipulates others to achieve his objectives, favoring psychological tactics over direct force—a talent valuable both in daily life and during missions undertaken with Ryoko. Outwardly calm and composed, this demeanor can become intimidating, amplifying his threatening presence. He possesses lethal combat skills matching Ryoko's dangerous capabilities.

Mikihiko maintains a multifaceted professional life reflecting diverse interests and resources, working as a cameraman filming Mayu and Shungo's training sessions, a farmer, a rancher, and a fishing boat operator. His relationship with Ryoko extends beyond partnership, often placing them in scenarios suggesting a battle couple dynamic, underpinned by their shared history. Beneath his manipulative exterior and occasional ruthlessness lies loyalty to his friends; he readily offers support and advice. This complexity paints him as a pragmatic yet principled figure dedicated to his sister's recovery and the stability of his inner circle.