Cocytus manifests as an insectoid humanoid towering at 2.5 meters, a fusion of mantis and ant characteristics. His physique bristles with sharp, icicle-like spikes and pale blue bone armor exuding cold air akin to diamond dust. Shoulders and back bear formations evoking uplifted icebergs, complemented by a tail twice his height. He serves as a Floor Guardian in his original world, overseeing martial defense with a strong warrior ethos. His personality blends unwavering loyalty to his master with a burgeoning sense of independent judgment. He respects opponents demonstrating fighting spirit or honorable conduct, valuing tenacity in battle. This leads him to initially dismiss characters like Subaru and Grantz as unworthy warriors, though he later revises his opinion of Subaru after observing his refusal to violate privacy. Conversely, he shows disdain for those perceived as lacking commitment, such as Kazuma Sato's reluctance during field day events. Within the school setting, Cocytus assumes leadership of the gym committee alongside Darkness, Weiss, Koenig, and Neumann, electing himself due to recognized physical prowess. He enforces rigorous training standards, demanding exemplary effort from committee members and classmates. He mandated one thousand push-ups during a beach trip, viewing Darkness as a positive example despite later sensing something amiss. His approach lacks flexibility; he labels intentional self-injury in dodgeball as idiotic and cannot excuse lax effort. His interactions reveal social awkwardness and observational gaps. While working clumsily at a craft store in front of students, he consistently finds Darkness perplexing, noting during science class that "something was not right" and suspecting he "missed something" about her during the beach session. Cocytus exhibits subtle dimensions beyond strict militarism. He demonstrates enjoyment in artistic activities like arts and crafts. He also displays a capacity for independent choice, selecting personal preferences over assumed alignment with his master during games, contrasting with more rigidly obedient peers. His background involves significant development; a major military defeat forced introspection, moving him beyond blind obedience towards offering strategic counsel and carefully weighing decisions to uphold his master's interests. He intensely fears failing his master, historically considering suicide to atone for dishonor. He harbors a desire to train his master's potential heir in combat, envisioning himself as a protective "cool uncle" figure safeguarding Nazarick's future.