Operator 21O is an android communication operator permanently stationed at the “Bunker”. Her duties include sending directives to the YoRHa soldiers on the field as well as information analysis. Personalities slightly differ between units.

Operator 21O, responsible for YoRHa unit No 9 Type S, is level-headed[1], cold, and impersonal. She shuns 9S's attempts at casual conversation and does not pay heed to his compliments of her. In terms of personality, she is a polar opposite to the much more outgoing Operator 6O. Despite her cold exterior, 21O harbors a secret longing for a family. She first hints at this as 9S completes sidequests for her that involve searching for relics of the old world, where she responds to the findings with interest where the relics pertain to human familial behavior.