Around twenty years before the events of the series, Midori Sako, her name at the time was part of a research team studying the relationship between devil transformation and brain function. She and her team were reassigned to the Hybrid Birth Project, where she first learned of its existence. Both she and her colleague Julius Kurtz were critical of the project’s methods and agreed that if they were ever promoted to executive positions, they would shut it down.

At the start of the fifteenth term of the Hybrid Birth Project, Midori met one of the sperm donors, Tamaki Anzai, a devil imprisoned in isolation for committing a mass murder. From their first meeting, she felt a mix of concern and curiosity toward him. Tamaki was kept in near-total isolation, mourning the loss of his former lover, and showed little interest in speaking with anyone. In an effort to improve his mental state, ONLO initiated “Dialogue Sessions,” with Midori serving as his primary contact. Over time, these sessions allowed Tamaki to open up for the first time since his lover’s death, and the two gradually formed a close bond. They eventually learned Japanese Sign Language together to make communication easier.

When the pairings for the project were announced, Tamaki was matched with Ibuki Takeya. While Tamaki expressed indifference to the selection, Midori surprised him by asking how he would feel if she were chosen instead. The conversation led Midori to realize she was in love with him. The question caused Tamaki enough emotional strain to fall ill, prompting Midori to visit him in the hospital, where she apologized for her bluntness. To her relief, Tamaki reciprocated her feelings. Upon learning that the project would use artificial insemination rather than physical intercourse, Tamaki expressed further comfort, believing such intimacy with a human to be impossible for him.

The ONLO administration approved a partner change under the condition that Tamaki’s sperm would be used to fertilize Midori’s eggs, with Ibuki Takeya acting as the surrogate. The arrangement was made both to safeguard Midori considered one of the organization’s most skilled researchers and to avoid reputational damage from directly associating a researcher with a convicted murderer. Ibuki consented, and in time, Midori and Tamaki’s son, Yuuki Anzai, was born. Because Yuuki was a subject of the Hybrid Birth Project, ONLO retained full custody, preventing Midori and Tamaki from having a legal parent-child relationship with him. Midori chose not to reveal the truth of his parentage, allowing Yuuki to believe he was an orphan, though she continued to watch over him from a distance while conducting her research on devils and their coexistence with humans.

Despite the challenges, Midori and Tamaki sought ways to maintain a shared life employing safety measures such as Midori carrying a firearm and tranquilizers, using handcuffs, or avoiding being alone together. Their commitment to one another eventually led them to marry.

Midori is a middle-aged woman with brown eyes, a light complexion, and a few visible age marks. She has wavy black hair that falls to her upper back, with long, middle-parted bangs framing her face. Two moles mark her features one beneath her eye and another on her cheek. Her son, Yuuki Anzai, closely resembles her, having inherited her smile, unruly black hair, and the same facial moles. Midori typically dresses in a professional manner, wearing a blouse paired with a white lab coat and heels.

After the series’ timeskip, she cuts her hair to chin length, with messy bangs framing her face. In her earlier years, she wore her hair short, approximately to nape length, with bangs covering her forehead.

Midori is a calm, responsible, and mature individual whose extensive knowledge and experience make her highly understanding of others. She is diligent in her work and accepts full responsibility for her actions, ensuring that personal feelings do not interfere with her professional duties. When emotions do influence her decisions, she promptly acknowledges her mistakes and offers sincere apologies. Patient and persevering, she prefers to let matters unfold at their own pace rather than rushing. Deeply passionate about her work, she approaches her research with seriousness and conviction, holding strong views on devil-human coexistence. Her decades-long relationship with Tamaki Anzai has given her significant firsthand insight into such relationships. Midori loves her family deeply and values their well-being above all else.

Alongside her professionalism, Midori has a teasing, playful side and often comes across as easygoing in personal interactions. She is notably honest and direct, speaking her mind regardless of how others might react. Comfortable discussing topics that others may find awkward or taboo, she is open and forthright in her communication, a trait remarked upon by both Tamaki and Yuuki.

The given name Midori (緑) means “green.” Her surname Anzai (安斎) combines an (安), meaning “peace” or “quiet,” with sai/zai (斎), which can mean “religious purification,” “Buddhist food,” “room,” “worship,” “avoid,” or “alike.” Her maiden name Sako (佐古) consists of sa (佐), meaning “help” or “aid,” and ko (古), meaning “old,” “antiquity,” or “ancient time.”