The character most directly referred to as Daughter in Death Note lacks a single definitive identity, as the series features several daughters of major supporting characters. The two most prominent figures fitting this description are Sayu Yagami, the younger sister of the protagonist Light Yagami and daughter of the police chief Soichiro Yagami, and Yumi Aizawa, the young daughter of detective Shuichi Aizawa.

Sayu Yagami is introduced as a cheerful and ordinary schoolgirl living in Tokyo. She is presented as a typical teenager who is not particularly intellectually gifted, especially in contrast to her genius brother, often relying on Light to help her with her math homework. Her personality is characterized by a kind-hearted, open, and somewhat frivolous nature. She develops a strong infatuation with the teen idol Hideki Ryuga and enjoys watching television. According to her father, Sayu is the type of person who would cry loudly even if someone she disliked were to die, highlighting her fundamentally empathetic and emotionally expressive temperament.

Sayu's role in the story is largely that of an innocent civilian who is tragically affected by the conflict between Kira and his pursuers. Initially, she represents the normalcy of family life that Light gradually distances himself from. Her most significant involvement in the plot occurs during the second half of the series when she is kidnapped by the mafia leader Mello. Mello uses her as a hostage to force the Japanese police to hand over the Death Note. This event leads to a desperate exchange where her father, Soichiro Yagami, trades the notebook for her safety. The trauma of the kidnapping has a devastating and permanent effect on Sayu. She suffers from severe psychological damage, leaving her unable to speak and confined to a wheelchair, effectively becoming a shell of her former self. By the end of the series, she is depicted as living without the ability to care for herself, a poignant symbol of the innocent lives shattered by the supernatural conflict.

Her key relationships are defined by her family. She shares a close bond with her father, Soichiro, and her mother, Sachiko. Her relationship with her brother Light is initially warm and affectionate, with Sayu looking up to him. However, as Light becomes increasingly consumed by his role as Kira, he grows distant from his family, and Sayu becomes more of a tool in his strategic calculations, though his internal monologues suggest a genuine, if complicated, unwillingness to kill her directly. Her development is a tragic arc from a bubbly, vivacious teenager to a mute, traumatized adult. By the age of twenty-one, she is depicted as having lost all her former vitality, a casualty of the war between Kira and those who oppose him.

In contrast, Yumi Aizawa is a much younger character with a minor but emotionally resonant role. She is the daughter of Shuichi Aizawa, a detective who works on the Kira investigation task force. Yumi appears as a small girl with short brown hair. Her primary function in the narrative is to humanize her father. In a pivotal scene, she finds her father sitting alone on a park bench after he has quit the task force out of frustration. When he explains he is taking time off, she innocently asks if this means he will be home more, leading her father to break down in tears as he realizes the personal cost of his obsession. Later in the series, after an older Aizawa has rejoined the investigation and begins to suspect Light Yagami, Yumi appears again, simply stating that she is studying and telling her father not to struggle so much. This highlights her growth and her continued role as a grounding presence in his life, contrasting the darkness of the investigation with the normality of home life. She possesses no special abilities, serving instead as a narrative device to explore the emotional stakes of the adult characters.