Katja, also known as Ekaterina Kurae, is a central figure in her story. She is a young girl of Russian origin who holds the title of the Qwaser of Copper, granting her the ability to manipulate this metal. Her youthful appearance is striking; with her blonde hair often tied into long pigtails and her gothic Lolita-style black dress, she projects an image of delicate doll-like beauty. This innocent look, however, is a carefully maintained facade.

Beneath a mask of shyness and childlike sweetness lies a true personality that is domineering, manipulative, and fundamentally sadistic. Katja is highly intelligent and prideful, viewing most people as tools to be used or dominated to serve her desires. She particularly enjoys exerting control over others, often through psychological manipulation and humiliation, a trait she channels into her relationships. Despite her outward cruelty, her character is shaped by a difficult past, including hints of abandonment and a lonely childhood that fostered a deep-seated distrust of others. She is also implied to be connected to the Romanov imperial family, a background that contributes to her sense of solitude and imperial pride, as well as a fierce hatred for the heretics who destroyed her family.

Katja’s primary motivation is the pursuit of her own desires and the fulfillment of her mission. She is sent by the Athos organization to her school, St. Mihailov Academy, as the second Qwaser. While she maintains a cold and ruthless exterior, she also harbors a hidden capacity for a certain kind of loyalty and protectiveness towards those who prove themselves genuinely loyal to her, even if she would never openly admit it. This is most clearly seen in her relationship with her copper puppet, Anastasia, whom she calls "mother." Her profound grief and sincere mourning after the puppet is destroyed reveal a deep, albeit rare, emotional vulnerability.

Her role in the story is that of a major protagonist and a deuteragonist, often acting as a rival to the iron Qwaser, Alexander "Sasha" Nikolaevich Hell. She frequently clashes with him, not necessarily as an enemy, but to assert her own superiority and wound his pride, resenting being treated as a child or someone in need of rescue. The most significant relationship in her life is with Hana Katsuragi. She effortlessly manipulated the older bully, turning her into an obedient slave and her primary source of soma, the breast milk that fuels a Qwaser's power. This relationship is characterized by strong S&M dynamics, which Katja uses to enhance the quality of the soma she extracts. Over time, she also comes to form partnerships with Lulu Shiizaki and Ayame Satsuki, bringing them under her sway as additional sources of soma. While generally dismissive of school life, she gradually opens up slightly to her classmates, developing a genuine, sisterly affection for the popular student Miyuri Tsujidou.

As a Qwaser, Katja’s notable abilities are all centered on her control over copper. She primarily channels this power through her gynoid puppet, Anastasia, manipulating it with extreme precision to fight on her behalf. Anastasia possesses immense strength, can channel powerful electrical currents through copper wires, and can even create new copper objects from scratch. While she is a formidable fighter through her puppet, Katja’s own physical abilities are unremarkable, making her heavily reliant on her unique metallurgical powers and the soma provided by her compliant partners to sustain them.