Kuro, alongside Shiro, serves as one of Masaki Andoh's two familiars. Despite their feline appearance, both firmly deny being actual cats. Their capacity for speech initially startles those who meet them, though individuals such as the *Hagane* crew accept this trait without extensive inquiry. Similar to Masaki, the origins and true nature of Kuro and Shiro remain a mystery, with their background and purpose largely undisclosed.

Kuro routinely provides advice and opinions on Masaki's actions and unfolding events, frequently critiquing his decisions or ridiculing his tendency to become lost. This dynamic features blunt assessments of Masaki's perceived shortcomings, often followed by expressions of concern for his safety, especially when his choices lead to trouble. In combat, Kuro offers direct support by functioning as a small mechanical drone. Together with Shiro, they operate the Cybuster's Familiar weapon system, launching energy projectiles from multiple angles during battles.

A notable exchange occurs when Kuro observes Sleigh Presty's combat style and red-themed mecha, likening her to the "Red Comet" due to her speed—a comparison promptly corrected by Shiro. This underscores their role as observers who frame events within established mecha conventions.

Throughout all appearances, Kuro maintains a consistent role centered on guidance, tactical combat support, and candid commentary on Masaki's actions. Their relationship with Masaki balances irreverent criticism with underlying loyalty and concern.