The Black Imperial Sword Judecca is a sentient Demon Lord Weapon originally sealed in the Sky Library before contracting with Arata Kasuga. Since then, Judecca has supported her master and eventualy became one of his primary weapons used in battles.

Upon being released from her seal, Judecca initially behaved similarly to an automated artifical intelligence system. However, after forming a contract with Arata, her responses gradually became more diverse and less robotic. As a Demon Lord Weapon, she will faithfully follow her master's desires and strive to fufill his wishes. In particular, she will even indulge with Arata's perversion.

After taking on her human form, she acts very affectionate and tries to flirt with him. However, she usually ignores all other people except her master.