Jita Phrygianos is a character from the anime The Qwaser of Stigmata, known by the code name Black Diamond and ranked as the Eleventh among the Adepts. She is the younger sister of Joshua Phrygianos, a fellow Adept whom she often openly derides as a fool, considering his perceived incompetence and elemental affinity a disgrace to their family. Despite this harsh judgment, she reacts defensively if others disparage him, suggesting a complex sibling dynamic where she reserves the right to criticize him for herself. Physically, she is depicted with neck-length black hair that has a slightly messy appearance, reddish-brown eyes, and typically wears a high-collared white coat over a short dress paired with black leggings and heeled boots.

Personality wise, Jita initially comes across as stern, harsh, and quick to judge. This is most evident in her first interactions with Tomo Yamanobe, whom she addresses as a loose woman based solely on her good looks. However, this abrasive exterior conceals deeper layers of insecurity and a capacity for genuine affection. She has a notable aversion to male touch, a condition known as androphobia, which causes her significant trauma if violated. She is also prone to becoming flustered and awkward, especially when confronted with nude females. Despite these vulnerabilities, she displays a strong sense of loyalty to her mentor, Wan Chen, a loyalty that persists even after his betrayal.

Her motivations and role in the story are heavily tied to her duties as an Adept scientist. In this capacity, she examines Tomo Yamanobe's body due to the dormant Qwaser of Gold within her, a task that quickly develops into an intense personal fascination. Her role often places her in conflict with other characters, and she is involved in key events such as appraising Tomo before a procedure intended to enhance her into a High Maria. She is also part of a tenuous alliance between the Adepts and the Ancient Regime as the plot advances toward the Tsar's Crowning.

Key relationships define much of her development. Her initial harsh treatment of Tomo transforms significantly after Tomo saves her when Georg Tanner forces her to take the blame for a fight he provoked. This act of defense causes Jita to slowly open up and develop a crush on Tomo. She also exhibits a gentler, protective side in her relationship with the timid Milk, a young Hydrogen Qwaser who rarely leaves her side. Her association with Georg Tanner is strained, as he distrusts both Phrygianos siblings and deems them inferior. Her fixation on Tomo also breaches the Adepts' conduct rules, creating internal conflict with her mentor. She is known to dislike Alexander Nikolaevich Hell, the series protagonist.

Jita undergoes notable character development, shifting from a solitary and judgmental figure to someone more open and capable of forming bonds. This change is primarily catalyzed by Tomo's kindness and bravery. She demonstrates a willingness to protect those she cares about, like Milk, and her previously rigid demeanor softens as she integrates more with others at the academy.

As the Qwaser of Carbon, Jita wields the ability to manipulate carbon atoms, which are abundant on Earth. Her primary combat technique involves forming carbon into bladed weapons, most notably a pair of crescent-like scythes. She can also create ultra-thin, durable diamond wires that can be used offensively or for restraint, as well as harden her own fingernails for close-quarters combat. A notable limitation of her ability is that diamond constructs, while extremely hard, can shatter under sufficient stress due to their crystalline structure. Her fighting style is visually reminiscent of another character codenamed Phoenix. She also struggles against non-physical attacks such as gases, radiation, or liquids. While not yet a master of her element by the end of the series, she shows steady progress in controlling her powers.