Kuroi Eri is a security professional and a member of the Kuroi Matriarchy, having been assigned the task of bodyguard to one Shizuki Ryouko. She had been sent by the Matriarchy to be a bodyguard for an increasingly important, though distant, member of the family. She compromised with the similarly charged Shizuki Elanis, member of the rival Kuroi Matriarchy, to share the task of bodyguarding Ryouko, though they were initially tense on matters of juristiction. Seemingly calmer than the firebrand Elanis, they would later on adjust to each other's presence, warming up to the task and becoming increasingly friendly with each other and their charges. It is implied that Eri and Elanis are sexually involved by the time of Ryouko's first visit to Adept Blue. They both continue to accompany Ryouko until her departure from the galaxy and are thereafter seen accompanying Nakihara Asami.