Nona

Nona manages the arbiter system from the ninetieth floor, the highest in the tower, and looks far younger than her decades of experience suggest. Calm and businesslike most of the time, she can also be sharply commanding, easily overpowering an arbiter twice her size when discipline is called for.

Her main project is Decim: she has quietly implanted him with human emotions to see whether an arbiter who actually feels something can judge more fairly than one who doesn't. To help the experiment along, she assigns a new assistant, later known as Chiyuki, to work at his side, and keeps the arrangement hidden from her superior, Oculus, for as long as she can.

Off duty, she unwinds with drinking partner Quin and games of billiards with Oculus, and relies on Clavis as a trusted aide who knows more about her plans than almost anyone else. She has little patience for the volatile arbiter Ginti, but is visibly protective of Decim even when she is disappointed in him.