Yuuki Anzai was born in Obihiro, Hokkaido, as part of the fifteenth term of the Hybrid Birth Project conducted by the Organization for National Living Organisms (ONLO). These hybrids, sometimes referred to as “Children of Light,” were created through controlled pairings between humans and devils. Anzai’s human mother, Midori Anzai, was a researcher for ONLO, while his father, Tamaki Anzai, was a devil and convicted serial killer. Unlike most pairings within the project, Midori and Tamaki were genuinely in love and used the program as a means to have a child together. Due to the nature of the experiment, Anzai was carried to term by a surrogate, Ibuki Takeya, and placed under ONLO custody immediately after birth, preventing his parents from gaining legal guardianship.

Unlike fellow hybrid Johannes Kleeman, Anzai was never subjected to direct experimentation. He lacked the accelerated healing ability common among hybrids, was physically frail, and exhibited difficulty with blood exposure, leading ONLO to deem him unsuitable for testing. Instead, he was placed in the organization’s orphanage for devil orphans, though his development was monitored for signs of ReMI (insufficient transformation capability). At the orphanage, he was only told that he was a hybrid and grew up believing he had no parents. He later described the ONLO staff and other children as his surrogate family. Among his closest companions were Tadaya Hiruta and Yuuko Yano, with whom he spent much of his childhood.

During a game of hide-and-seek, a young Anzai became stuck in a tree and was helped down by Kirio Kikuhara, a visiting detective. The two quickly became acquainted, and Kikuhara told Anzai that he wished for someone like him to be his comrade. Inviting Anzai to “follow” him, Kikuhara led the boy to ONLO’s underground detention facility, where numerous devils who had committed violent crimes were being held for research. There, Kikuhara revealed that all devils were essentially the same, implying Anzai was no different despite being a hybrid. He then took Anzai to meet his father, explaining his own past and drawing a parallel to Tamaki’s crimes, which included the murder of fifteen people. The revelation overwhelmed Anzai, causing him to cry and vomit. The traumatic encounter instilled in him a deep resentment toward devils, though he later suppressed the memory due to its intensity.

In his later years at ONLO High School, Anzai resolved to become a police officer alongside Tadaya, aspiring to protect civilians from devil-related threats. After graduation, he was assigned to the Tokyo Metropolitan Police Department’s Public Safety Division 5, joining the F Squad as a specialist in devil-related cases.

Yuuki Anzai is a young man of average height, standing at 176 cm, with a lean yet muscular build. He has light blue eyes and short, tousled black hair, distinguished by a longer strand that falls over his forehead, a feature he retains even after getting a haircut during the timeskip. A small beauty mark rests just above the left side of his upper lip. Anzai inherited his black hair, smile, and beauty mark from his mother, Midori, though his facial structure strongly resembles that of his father, Tamaki. As a devil who abstains from drinking blood, he bears noticeable dark circles beneath his eyes.

His typical attire consists of a black shirt, light-colored trousers, and dress shoes. He is consistently seen wearing a silver ring adorned with a black cross on the middle finger of his right hand; the ring doubles as a device for monitoring his body temperature.

When Anzai begins to transform into his devil form, the change follows a distinct progression. His eyes first darken to red, after which his irises shift to a golden hue with slit like pupils. Veins become visible around his eyes, his teeth elongate into fangs, and his fingernails sharpen into claws. In this state, his physical abilities including strength, speed, and reflexes are significantly heightened. The pace of transformation varies depending on the trigger: emotional arousal typically causes a gradual shift, contact with blood accelerates the change, and life-threatening situations prompt an almost instantaneous transformation.

Yuuki Anzai initially presents as a reserved and enigmatic individual, yet beneath this exterior lies a strong moral compass and a deep sense of justice. His commitment to protecting civilians from devils is a core reason for joining the police force. Typically calm and composed, Anzai can become fiercely aggressive when those he cares for particularly Tsukasa Taira are threatened. He is quick to intervene on behalf of friends and comrades, as shown when he transformed instantly after Tsukasa was shot, reacting with visible relief upon learning she was unharmed. While he seldom smiles, he becomes noticeably more open and warm in the company of close friends such as Tadaya Hiruta or when sharing moments with Tsukasa. By contrast, toward adversaries or individuals he distrusts, Anzai adopts a cold, blunt attitude.

Although mature and responsible, Anzai occasionally acts on impulse, allowing emotions to override reason. Johannes Kleeman has described him as emotionally susceptible projecting toughness while being inherently gentle. Anzai is inclined to empathize even with those who have wronged him, often attributing their actions to personal hardship. This tendency is evident in his refusal to hate Nanako Tenjo despite her attempt on his life, understanding her deep resentment toward devils. Rather than confront conflicts directly, he tends to internalize blame and avoid discussion. He rarely holds grudges in personal matters and will readily apologize when in the wrong, but he remains protective and resentful toward anyone who harms Tsukasa without remorse.

Anzai struggles with self-acceptance, harboring deep resentment toward his devil half and viewing his transformed state as monstrous. This self-perception, born of his own judgment rather than external criticism, initially made him hesitant to be close to Tsukasa. Over time, her acceptance and support challenged his pessimism toward human-devil coexistence, gradually improving his self-esteem. Despite this progress, he temporarily ended their relationship to resolve his inner conflict and spare her from danger, later reconciling with the resolve to remain by each other’s side.

Tsukasa is the only person who can fluster Anzai, particularly when she expresses affection or jealousy. Most of his transformations occur in situations involving her, often driven by romantic or physical attraction. On at least one occasion, these urges led to private acts following their first bloodlust training session. Outside of serious situations, Anzai also exhibits a humorous side, occasionally engaging in petty arguments or lighthearted exchanges with Lee, whose personality contrasts sharply with his own.

Fighting Skills and Strength
Anzai is a highly capable police officer, known for his agility, speed, and precise movement. He is able to disappear and reappear in an instant, and can leap to heights comparable to telephone poles. While his physical abilities are notable, he typically refrains from using his full strength to avoid injuring opponents and causing them to bleed. In his untransformed state, this restraint leaves him vulnerable to defeat by other devils or skilled humans, such as Megumi Ishimaru.

In his transformed state, Anzai’s combat potential increases dramatically. He is able to exert nearly the full extent of his muscle strength, achieving greater speed, sharper senses, and faster reaction times. These enhancements allow him to dispatch multiple opponents consecutively and evade incoming attacks with ease, though he is occasionally prone to lowering his guard during engagements.

Transformation and Healing
Prior to meeting Tsukasa, Anzai’s bloodlust was below average, and he was able to suppress it entirely, avoiding blood consumption for 21 years. His transformations were classified as insufficient (ReMI), meaning they lacked the complete loss of control typical in most devils. His condition, described by his mother Midori as “indefinite ReMI,” refers to a transformation triggered by specific emotional or physical stimuli.

Meeting Tsukasa awakened stronger impulses, including sexual attraction linked to his bloodlust. This awakening intensified his transformations, making them more physically extreme and mentally dangerous, often causing him to lose control. In heightened situations, his fangs and claws can grow to extraordinary lengths, and his actions can become erratic and destructive. His healing ability also emerged during this period, enabling him to recover from life-threatening injuries when supplied with sufficient blood.

Anzai’s primary trigger for transformation is anger particularly when Tsukasa is harmed though lust can also act as a catalyst. Over time, he has developed partial control, occasionally halting his transformation mid-process so that only the upper halves of his eyes change. He has identified two conditions for maintaining stability: controlling his anger and receiving forgiveness from Tsukasa. However, this control remains fragile, and when desire overpowers restraint, he quickly reverts to a full transformation.

The given name Yuuki (結貴) is composed of the kanji yuu (結), meaning “tie” or “bind,” and ki (貴), meaning “valuable.” His surname Anzai (安斎) combines an (安), meaning “peace” or “quiet,” with sai/zai (斎), which can refer to “religious purification,” “Buddhist food,” “room,” “worship,” “avoid,” or “alike.”