Sung Jinah, also known as Aoi Mizushino in the Japanese version of the anime, is a major supporting character in the story. She is introduced as the younger sister of the protagonist, Sung Jinwoo, and serves as a central figure in his personal life, representing the ordinary world and family bonds he is determined to protect.

In terms of background, Jinah is a high school student, approximately seventeen years old, who lives with her brother following their father's disappearance and their mother's hospitalization due to a mysterious illness called Eternal Slumber. With both parents absent, Jinwoo took on the role of her guardian and primary provider, a responsibility that deeply shapes their relationship. Jinah, in turn, works hard to support her brother by excelling academically, maintaining top marks throughout her schooling.

Jinah is characterized by a cheerful, energetic, and straightforward personality. She is depicted as bright and diligent, with a strong sense of responsibility that belies her age. Despite the constant danger her brother faces as a hunter, she maintains an optimistic and caring demeanor, though she is deeply worried about his well-being. She is known for treating her brother casually and without fear, even as his power and appearance undergo dramatic and unsettling changes. This natural, grounding attitude provides Jinwoo with a stable and normal emotional anchor amid his increasingly dangerous life. A recurring trait is her habit of playfully kicking her brother when annoyed, only to end up hurting herself due to his superhuman durability.

Her primary motivation is to secure a stable and hopeful future for herself and her family. Jinah dreams of entering a medical university and becoming a doctor, a goal directly inspired by her mother's long-term illness. She hopes that by achieving this, she can help heal her mother and alleviate the burdens her brother has carried for so long. Her narrative role is to personify the normal, peaceful life that Jinwoo fights to preserve. Her presence constantly reminds the audience of his human ties and what is at stake in his battles against monsters and other hunters.

Key relationships define her place in the story. Her most significant bond is with her brother, Sung Jinwoo. She worries about him constantly, especially when he returns home injured from dungeon raids, and her faith in him remains unwavering. Her devotion to her hospitalized mother is also a defining trait; she makes regular visits to the hospital, fitting them into her busy study schedule, which underscores her maturity and deep family loyalty. Through her school life, Jinah is connected to the world of ordinary people, providing a stark contrast to the violent and chaotic realm of hunters that her brother inhabits.

Throughout the story, Jinah develops from a worried teenage girl into a more resolute young woman. Though she is initially horrified by her brother's altered appearance and newfound intensity, she quickly overcomes her shock and chooses to treat him with the same warmth and familiarity as always, demonstrating the strength and resilience of their familial love. Her own personal development is focused on her academic journey and her unwavering commitment to her dream of becoming a doctor, symbolizing a path of healing and creation that contrasts with Jinwoo's path of combat.

Unlike the hunters who populate much of the series, Jinah possesses no notable combat abilities or magical powers. Her strengths lie in her intelligence, discipline, and emotional fortitude. Her excellent academic performance and her ambition to enter medical school are her primary means of contributing to her family's future, showcasing a different kind of strength that is vital to the story's emotional core.