Julia Katsushika is a lonely individual driven by a strong desire for love, leading to a submissive and vulnerable nature. She previously worked as a club hostess. Her first relationship was with club host Seiya Shinjuku, living together in a shared apartment. This ended when Seiya became Ayaka Toshima's slave, forcing him to break up with Julia and expel her from their home.

Following the breakup, Julia became Ryuuou Edogawa's nanny, unaware he orchestrated this to manipulate her into wearing an SCM device. Ryuuou tricked her into using the SCM, making her his first slave. Despite her enslavement, Julia developed genuine feelings for Ryuuou; it is strongly implied she would have willingly done anything for him without the SCM's influence.

During a later encounter with Seiya, Julia rejected his reconciliation proposal, revealing she was now Ryuuou's slave. She used the SCM's "unaware opponent" rule to trick Seiya into a game, temporarily making him her slave before transferring control to Ryuuou.

Julia became pregnant. While Fujiko Taito initially claimed the child was Ryuuou's, Julia clarified it was Seiya's. She gave birth to Seiya's child and maintained a healthy lifestyle afterward, though raising the child proved challenging. The baby's gender was never disclosed.

Julia possesses thigh-length caramel-brown hair, featuring distinct chest-length strands framing either side of her face. She has dark-brown eyes, full pink lips, and large breasts.

Her name "Julia" derives from Latin, meaning "youthful." Her surname "Katsushika" combines the characters for "arrowroot" and "decoration."