Operator 21O (pronounced Operator two-one-oh) 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 No.9 Type S, is level-headed, 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.
|That is why you, the YoRHa forces, exist:|
SPOILER ALERT - Plot details for NieR: Automata follow.
In the 9S's mission to disable Machine defenses at the start of the third game cycle, if 9S fully deactivate the air defense (by hacking the Goliath-class robot at the supermarket entrance), 21O will offer to 9S a gift that she put in her room.
After monitoring this mission, 21O is not heard from again until much later, when 9S infiltrates the God Box resource station. There, it is revealed that 21O voluntarily converted into a B unit, becoming 21B for the YoRHa mass assault that was thwarted by the logic virus outbreak. Infected by the virus herself, 21O struggles as she fights 9S, pleading that she just wanted to be part of a family, hinted at in her previous quests, and then for 9S to kill her.
21O is killed when A2 appears from behind and runs her through. Though A2 manages to grant 21O's wish to die and saves 9S, the event only furthers 9S's mental breakdown.