"Rui" is an individual who has been administered a mysterious drug concocted by the yakuza group Garyukai, which amplifies his reflexes, agility, and speed to extraordinary extents. He exhibits mercilessness when it comes to eliminating people but transforms into a trembling wreck when scolded by Tsukasa. When not under the influence of the drug, Rui possesses a subdued and shy demeanor.
Having been raised without a mother, Rui developed a strong attachment to his father, often joining him on various botanical expeditions. During one such trip to India, they stumbled upon the "kagi" and a map indicating the location of the plant used in creating the drug. His father shared this discovery with an academic board that included a member affiliated with Garyukai, inadvertently leading to the secret's exposure to them.
Rui, while influenced by the drug, is responsible for severing Asago's right arm.
Note: The translation inaccurately identifies Rui as female in several instances.
