Saeki Akari, the heiress to the Asu Pharmaceutical Group, has a "guardian spirit" named [Mercy].
A lick of her tongue can heal external injuries, and her saliva can cure illness. This power has made her the cornerstone of her adoptive mother's business empire, and also left her accustomed to being surrounded by bodyguards and hiding behind the sharp-tongued, haughty airs of a young lady.
She had already planned out her future: go to college with her close friend Kakyoin Noriaki, who possesses the same unusual ability, and gradually learn to manage the company.
Then one day, Kakyoin suddenly transferred schools without saying goodbye.
Following the clues to his new school, she came face-to-face with a man as tall as a mountain, his cap brim pulled low, carrying her unconscious best friend on his back. In that moment, Akari thought Kakyoin had fallen victim to a delinquent.
She had no idea she was about to set foot on a journey of fate spanning a century!
Theme: True mercy is never afraid to get blood on its hands.
