Asia Argento is a central figure in the narrative, originally introduced as a former church修女 whose life changes dramatically after encountering the protagonist. Her background is marked by tragedy and a deep sense of isolation. Born in Northern Europe, she was abandoned as a baby and subsequently taken in and raised by the church, becoming a修女. Within the church, she was revered as a圣女 due to her unique ability to heal any wound or illness using her Sacred Gear, Twilight Healing. Ironically, her pure and indiscriminate use of this power to heal a Devil led to her being branded a heretic and excommunicated. This event set her on a path that ended with her being taken in by Fallen Angels, who ultimately killed her to steal her Sacred Gear. She is given a second chance at life when she is reincarnated as a Devil by Rias Gremory, becoming a member of Rias's peerage as a Bishop.

In terms of personality, Asia is defined by her profound kindness, innocence, and purity of heart. Having grown up in a sheltered religious environment without a family or friends, she is initially very shy, socially inexperienced, and possesses a naively trusting nature. Despite the church’s betrayal and her subsequent reincarnation as a Devil, she clings to her faith in God and continues to cherish the teachings of the Bible, even though it causes her physical pain to do so. Her compassion extends to all beings, regardless of their race, and she refuses to discriminate against Devils, Angels, or Fallen Angels. This gentle demeanor, however, belies a quiet inner strength; she can be fiercely protective of those she loves and is not afraid to express anger, such as when she slaps a villain for insulting a dear friend. Her character is also marked by a charming lack of common sense and a comical tendency to be clumsy, often tripping on flat ground.

Her primary motivation evolves from a simple desire for belonging to a fierce dedication to protect her new family. Having spent her life as an orphan who was worshipped from a distance but never truly accepted, her greatest wish is to have a place where she belongs. After being saved and befriended by Issei Hyoudou, the protagonist, and then reincarnated by Rias Gremory, she finds that home within the Gremory group. Her actions are therefore almost entirely driven by loyalty and love for her new companions, especially Issei, whom she sees as her savior and develops a deep romantic affection for. She willingly puts herself in danger to heal and support her friends in battle, viewing her ability to serve them as a source of happiness and purpose.

Within the story, Asia serves as the primary healer for the group, a role of immense tactical importance that often places her at the center of major conflicts. Her presence is a key emotional anchor for the protagonist, helping him overcome his own traumas and anxieties. Her key relationships are the driving force of her character development. Her bond with Issei Hyoudou is the most significant; her love for him is unwavering and pure, and she is often a source of emotional support and gentle scolding when his lecherous tendencies get out of hand. She shares a deep, sisterly bond with her master, Rias Gremory, whom she greatly admires and respects as an older sister figure. Along with Xenovia Quarta and Irina Shidou, she forms the教会三人组, finding in them the peer friendships she was denied as a修女.

Throughout the series, Asia undergoes significant development, growing from a timid, passive victim into a more confident and assertive individual. Initially a character who is acted upon by the forces around her, she gradually finds her own voice and agency. This is partly due to the protective environment of her new family, but also due to her own resolve to stand by her loved ones. A major turning point in her development is her mastery of her own abilities. She discovers a natural talent as a Dragon Tamer, forming a契約 with the legendary Dragon King, Fafnir. This pact allows her to develop a powerful subspecies Balance Breaker called the Holy Dragon Princess's Garden of Affection, which greatly enhances her healing capabilities and provides defensive support for her allies. By the end of the series, she has matured into a leader, becoming the president of the Occult Research Club in her third year of high school, and eventually marrying the protagonist and starting a family.

Asia’s notable abilities center almost entirely on support rather than direct offense. Her primary power is her Sacred Gear, Twilight Healing, also known as the圣母的微笑. This gear grants her the unparalleled ability to heal the wounds and ailments of any living being, whether they be human, Devil, Fallen Angel, or Angel. She can use this power through direct touch or by projecting it as a projectile. Later in the story, after receiving a blessing from the infinite Dragon God Ophis, she gains the ability to form a deep bond with dragons. This leads to her契约 with the Dragon King Fafnir, who can be summoned to fight by her side. The most potent expression of her power is her subspecies Balance Breaker, which surrounds her in golden armor and creates a large area-of-effect zone that continuously heals her allies while also providing them with defensive buffs.