Sayu Yagami

Sayu Yagami (夜神 粧裕, Yagami Sayu) is Light Yagami's kindhearted younger sister and plays a minor role in the main storyline, remaining unaware of Light's possession of the Death Note. Sayu, born on June 18, 1989, is a 14-year old second-year student (8th grade) at Eishu Junior High School (英集中学校, Eishu Chūgakkō) at the beginning of the story. Sayu asks Light for help with mathematic homework, including quadratic function assignments, and looks up to Light as she sees his intelligence and believes that he will become a quality detective.

In the second part of the series, she is kidnapped by Mello's gang and taken to the United States, but is later returned to her father, Soichiro Yagami, when he gives the gang the Death Note. Because of these events, in both the manga and anime, she falls into a state of shock and becomes unresponsive to everyone, including her family.

In Chapter 97 of the manga, Sayu is last seen in one panel at an Open Day Ceremony. It is up to the reader to decide what her mental state is. Tsugumi Obha said that she hasn't "necessarily recovered but you could not tell from the picture".

Sayu in the film
Sayu, portrayed by Hikari Mitsushima, is a high school student, is not kidnapped (the mafia play no role in the two films she appears in) and remains ignorant of Light's role as Kira, instead being told that he was killed for his role in the investigation team. In "The Last Name", Sayu appears at the Sakura TV festival and causes a small incident when Kanzo Mogi dies and unknowingly temporarily shocks Misa. The incident results in having Soichiro and Light to pick her up leading to Misa chancing upon Light with her shinigami eyes. At the end of the movie, Sayu is sent by Sachiko Yagami to pick up Soichiro from the train station. They discuss how crime rates have risen following the death of Kira, and when Soichiro asks about her views on the subject, she states that she could never support him, particularly after the death of Light.