Hyo Shimaki serves as the Black Cat Knight alongside his familiar Coo Ritter. As a university professor specializing in psychological research, his initial defining trait is an obsessive pursuit of knowledge, specifically aiming to access the Akashic records—the theoretical compendium of all universal knowledge. This intellectual ambition draws the attention of Animus, the series' antagonist, who recognizes their shared goals and attempts to recruit him. Animus offers Shimaki the chance to explore the universe in exchange for defection, presenting a critical moral dilemma. Shimaki seriously contemplates this proposal during dream-world encounters but ultimately rejects it and attacks Animus, revealing his allegiance to the Beast Knights despite earlier self-centered tendencies.

His contractual wish for omniscience is denied by Coo. Instead, Shimaki gains ultimate conceptualization through Domain Control - Gaia, enabling him to create and command golems. These constructs bear names derived from German days of the week initially, later shifting to German seasonal terms. His golems exhibit escalating destructive capacity, evolving from building-level to small city-level potency. They play decisive roles in major conflicts, including toppling the mountain-sized golem Poseidon alongside Ende. This progression sparks ally concerns that he could eventually forge a golem rivaling Princess Samidare’s power.

Personality-wise, Shimaki shifts from quiet, mysterious, and academically detached to demonstrating authentic concern for fellow knights. His refusal of Animus’ offer marks a pivotal turn toward valuing interpersonal bonds over abstract knowledge. This evolution culminates in his marriage to fellow knight Yayoi Hakudou, disclosed a decade after the final battle. He explicitly dislikes alcohol.

Physically, Shimaki possesses baseline human capabilities without golem support, including human-level durability and lifting strength. His reactions are superhuman, and his golems operate at subsonic speeds. He exhibits superhuman stamina for sustained psychic exertion. Intellect remains his defining attribute, featuring mastery in psychology and extensive theoretical physics knowledge.