Li Xiangying, also known by her codename Glass Heart, is the central protagonist of Angel Heart. She is a young woman of Taiwanese origin who was separated from her family at a very early age. At around two years old, she and her mother were involved in a car accident that killed her mother and left her missing. Unbeknownst to her father, the head of the Taiwanese crime syndicate Zheng Dao Hui, she was taken in by the Zhuque Corps, a specialized assassination division within the same organization. Raised without any memory of her early childhood, she was trained from childhood to be a professional killer, referred to only as Number 27 during her training.

After completing her training, she was assigned the codename Glass Heart and became one of the syndicate's most effective assassins. However, the psychological toll of her work became unbearable after she killed a man while his young daughter watched. Overcome by guilt and despair, she attempted suicide by jumping off a building. She survived the fall but suffered catastrophic chest injuries. To save her life and preserve a valuable asset, the organization arranged for an urgent heart transplant. The heart she received belonged to Kaori Makimura, a young woman who had been declared brain dead after a traffic accident in Tokyo. The transplant was performed using a heart that had been stolen during transport from the hospital.

Li Xiangying spent about a year in a coma in Taiwan following the transplant. During that time, she experienced fragmented visions and emotions that were not her own, including memories of a man she did not recognize and an overwhelming sense of connection to a life she had never lived. When she finally awoke, she was driven by these borrowed memories to travel to Tokyo's Shinjuku district to find the man she had seen, Ryo Saeba. After a series of encounters, Ryo recognized that she carried Kaori's heart, and he chose to adopt her as his and Kaori's daughter. Her biological father, Li Jianqiang, also gave her the name Xiang Ying, which incorporates the Chinese pronunciation of the character for Kaori's name.

As a character, Li Xiangying initially displays a personality shaped entirely by her upbringing as an assassin. She is quiet, observant, efficient, and deadly. Having had almost no contact with ordinary society, she interprets most situations through the lens of threat assessment and responds with immediate physical force. She is completely unfamiliar with everyday social norms and struggles to understand casual interaction. Over time, however, Kaori's heart begins to exert a profound influence on her behavior. She inherits not only Kaori's memories and emotions but also her temperament, including her fierce loyalty, her short temper, and her tendency to resort to oversized hammers and other improvised weapons when angered. This marks a stark departure from her previous training, which emphasized stealth and precision.

Her primary motivation throughout the story is to find a place where she belongs and to understand the strange second chance she has been given. She is initially driven by the borrowed longing for Ryo that came with Kaori's heart, but this evolves into a genuine bond as Ryo takes on the role of a protective father figure. She also seeks to reconcile her past as a killer with her desire to live a peaceful life. The conflict between her assassin conditioning and her emerging gentle impulses is a central source of drama in her arc.

In the narrative, Li Xiangying acts as both the emotional anchor and the catalyst for change. Her arrival forces Ryo to emerge from his grief and resume living, and her presence reshapes the relationships among the existing cast, including the streetwise police commissioner Saeko Nogami, the blind former mercenary Umibozu, and other allies. She also serves as the bridge between the world of organized crime in Taiwan and the underground cleanup business that Ryo once operated.

Her key relationships include her adoptive father Ryo Saeba, whom she initially sought out because of Kaori's memories and whom she comes to love as family. Her bond with her biological father Li Jianqiang is more complicated, as they are reunited only after years of separation and after she has already been shaped by forces outside his control. Another significant relationship is with Liu Xinhong, a fellow trainee from the Zhuque Corps who harbors deep feelings for her and follows her to Tokyo. Her relationship with the memory of Kaori is equally crucial, as Kaori's spirit occasionally appears in reflections and moments of emotional intensity, guiding her or offering comfort.

Her development over the course of the story is remarkable. She begins as a nearly emotionless survivor of extreme conditioning and gradually learns to laugh, cry, feel guilt, and form genuine attachments. She learns how to function in civilian life, how to control her violent reflexes, and how to channel her formidable skills toward protecting others rather than killing. By the later stages, she has become a more rounded and emotionally open person, though she never fully loses the sharp instincts of her training.

In terms of notable abilities, Li Xiangying is an extraordinarily skilled fighter and assassin. Her training in the Zhuque Corps made her proficient in hand-to-hand combat, firearms, and stealth elimination. She is highly athletic, agile, and capable of improvising weapons from her surroundings. After receiving Kaori's heart, she also gains a curious and often comedic ability to produce massive hammers and other blunt objects from seemingly nowhere, a trait that closely mirrors Kaori's own habit of physically disciplining Ryo. This ability is tied to the emotional inheritance from the heart rather than any formal training. She also has a high tolerance for pain and a survival instinct that was reinforced by her years as an operative. Despite her young age, she is treated as a serious threat by both allies and enemies.