Arumi Nikaidō pilots mechs for Namidome Industries, combating giant beasts infesting Ryugan Island with machines like the Bullbuster and Bullrover. She maintains an aloof and unfriendly demeanor, often distancing herself from colleagues and preferring solitude during duties. Despite this reserved exterior, she reveals significant emotional vulnerability in intense situations—notably breaking down upon reuniting with a mutated creature named Shiro and again when forced to euthanize it during an urban battle. Her role demands close coordination with fellow pilots Tetsurō Okino and Ginnosuke Mutō, especially during missions deploying or repairing company mechs against giant beast threats. Though her background before Namidome Industries remains undocumented, her development through operations shows a character balancing professional detachment with profound emotional exposure tied to mission outcomes and personal connections formed during assignments.