Kuro is a contracted spirit of darkness who takes the form of a black fox and is a near-constant companion to Bertia, the protagonist. In her humanoid appearance she presents as a young girl with black shoulder-length hair and purple eyes. As a being of darkness, her primary abilities are defensive in nature, allowing her to shield Bertia and aid in warding off threats. Her initial bond with Bertia is rooted in the obligations of a spirit contract, but over time she develops a genuine protective attachment that goes beyond simple duty. She also forms a meaningful personal connection with Zeno, a spirit who serves the crown prince Cecil, and eventually chooses him as her mate. This evolution adds an emotional dimension to her character, showing her capacity for companionship beyond her formal role. Within the story, Kuro serves as a quiet, stalwart presence, using her defensive magic to support Bertia while gradually building her own ties to the people around her.