Jita Phrygianos is a character from the anime and manga series The Qwaser of Stigmata, where she holds the position of the Eleventh of the Adepts and is known by the codename Black Diamond. At fifteen years old, she has neck-length black hair and reddish-brown eyes. Her typical attire consists of a long, high-collared white coat reminiscent of a lab suit or formal coat, worn over a short dress resembling a maid's uniform, complemented by black leggings and heeled boots. She is the younger sister of Joshua Phrygianos, a relationship she often navigates by openly deriding him as a fool, particularly because of what she perceives as his incompetence and his elemental affinity; however, she reacts defensively if others speak poorly of him, suggesting a more complex, if guarded, family dynamic.

Jita possesses a serious and stern personality, marked by a deep sense of loyalty to her mentor, Wan Chen, a devotion that persists even after his betrayal. Despite her composed and sometimes harsh exterior, she has notable vulnerabilities, including a tendency to become flustered around naked women and a pronounced androphobia, an intense aversion to being touched by men. Her role within the story is tied to her membership in the Adepts, an organization of Qwasers. Her primary motivation and a significant part of her character arc involve Tomo Yamanobe. Initially, Jita regards Tomo with harsh judgment, calling her a loose woman due to her mature looks and large breasts, and she develops a strong, scientific fascination with Tomo's body because it hosts the dormant Qwaser of Gold. In one instance, she is shown appraising Tomo prior to a procedure intended to transform her into a High Maria.

However, this initial harshness transforms when Tomo saves Jita from being forced to take the blame for a fight provoked by Georg Tanner. Following this event, Jita opens up and develops what is described as a crush on Tomo, showing a softer, more vulnerable side. This key relationship is central to her development, as she slowly begins to warm up to others at St. Mihailov Academy, including students like Fumika, and shows gentleness in her protective relationship with the timid character Milk. Her loyalties are tested by internal conflicts within the Adepts, as Georg Tanner distrusts the Phrygianos siblings, and her own fixation on Tomo breaches the organization's conduct rules.

As the Qwaser of Carbon, Jita has the ability to manipulate carbon atoms, an element abundant on Earth, which allows her to source material for her powers with relative ease. She typically manifests this ability by creating bladed weapons, such as a pair of crescent-like scythes, or by forming ultra-thin, durable fiber threads of diamond. Her technique using diamond wires is visually compared to the Phoenix's fighting style. For close-quarters combat, she can harden her own fingernails to serve as weapons. Physically, her capabilities are above average, demonstrated when she showcased increased agility by leaping over the walls of the academy in an escape attempt. Her abilities are not without limitations; her diamond constructs have a crystalline structure that can shatter under sufficient stress, and she has difficulty countering non-physical attacks such as gases, radiation, or liquids. She has not yet mastered her element, though she is shown to be making steady progress with it in the manga source material.