Yui Hirasaka operates as an antagonist within a research organization, commanding specialized military units called War Dogs. Her mission focuses on apprehending the escaped research subject Poem. Utilizing neural-linked nanomachines (NLN) technology, Yui manipulates machinery through willpower alone—a capability identical to Shiki Mikage’s direct control over mechanical systems.

During a pursuit of Remu Mikage and Poem, Yui deploys her War Dogs to capture them. Remu counters by simultaneously disrupting her optical nerve via NLN interference and disabling the War Dogs’ optical sensors, temporarily blinding her. Yui adapts by switching to infrared detection to track her targets, confident in its effectiveness. Remu reduces his infrared visibility by lowering his body temperature with cold water, while Poem initiates a separate attack. Confronted by this sustained resistance, Yui employs a flash grenade to retreat.