Ruka Kiyose is a Japanese female characterized by light blue hair and olive eyes. She acts as personal servant to Hotaruko on a secluded island. During Ryo Inasa's, Hotaruko's cousin, residence on the island, Ruka forms a deep bond with Ryo, becoming his best friend and lover. Her character exhibits fierce loyalty and protectiveness toward Ryo. Witnessing the island's dark rituals and cult activities compels her to attempt facilitating Ryo's escape. Her relationship with Ryo is central to her actions, though Ryo ultimately rejects her pleas and chooses to remain with Hotaruko.

Ruka endures repeated non-consensual encounters with the island's high priest, Master Komoshi, who suggests these acts have a history. In the climax, she secures a special sword designed to kill the cult's god-like figure, Master Mizuno, intending to free Ryo from the island's influence. After successfully using the sword to destroy Mizuno, Ruka is immediately killed by Ryo. Ryo, driven to insanity following Mizuno's demise, delivers the fatal blow. Ruka's death concludes her arc abruptly. Her actions directly precipitate the climax but result in her demise at the hands of her lover.