Rock Vulcan directs the Gaist Crusher Garrison (GCG), overseeing strategy and personnel against the metallic Gaist threat. Previously, he fought as a frontline Gaist Crusher until the "Zeus Incident" nine years before the main story, which stripped his ability to perform "Gaist On" and use Gaist Gear armor, forcing his shift to administration.

During a Gaist attack on a hometown, his failed rescue mission led to Honoka's death while attempting to save her and sibling Izuna Kokuyou. An orphanage staff member brainwashed Izuna into blaming Vulcan for Honoka's loss, fueling lasting resentment. This incident spawned leadership rumors and strained their dynamic, driving Izuna to join the antagonistic Erasers before later reconciling with the GCG.

Physically, Vulcan is tall with spiked dark hair, a small goatee, and rectangular red eyes featuring downward-pointed eyebrows. He wears a light-blue jacket with orange lining over a black shirt and pants, complemented by light-grey boots and bronze shoulder plates.

His leadership blends approachability with authority, often joking with colleagues yet turning serious in crises. This duality includes recurring "Delayed Reaction" moments and brief withdrawn states. Episode 36 teases implied romantic tension between him and Luminella Hotaru, head of the GCG Science Division.