Koyori Akutagawa, younger twin sister of Mayumi Akutagawa and daughter of Dr. and Mrs. Akutagawa, is a Japanese national originally transferring from Brazil. Placed in an artificial coma to preserve her life force prior to becoming a Hand Shaker, her condition shifted dramatically when Tazuna Takatsuki grasped her hand during a university research facility visit. This contact awakened her, forging the Hand Shaker partnership Team Gear and becoming vital to her survival; breaking contact risked destabilizing her lifespan.

Initially expressionless and largely non-verbal, communicating primarily through actions, Koyori proved unable to summon her personal Nimrod—a weapon drawn from a Hand Shaker's psyche. She succeeded in manifesting it during a battle in Episode 9, earning the code name "Sprocket Gear" from Hibiki Moriyama. Despite her quiet demeanor, she displayed deep compassion and care for Tazuna, often reminding him of his deceased younger sister, Musubu Takatsuki. Her survival was later tied to Musubu's prior organ donation authorization.

Roughly ten years after the confrontation with "God," Koyori reappeared as a researcher at Kita Asuka Academy. By then, she had lost her Hand Shaker abilities, unable to summon Nimrods or access the Ziggurat dimension. Her physical appearance indicated halted aging, likely a consequence of her encounter with "God." During this time, she actively sought information on the whereabouts of her missing twin sister, Mayumi, and Mayumi's Hand Shaker partner, Nagaoka. She encountered the irregular Hand Shaker Yukiya Araki, advising him of his dangers while requesting his aid in locating Mayumi and Nagaoka.

Following Yukiya's successful wish to "God" near the events' conclusion, Koyori's aging process resumed normally. She ultimately married her longtime partner, Tazuna Takatsuki.