Sayu Yagami

Sayu Yagami (夜神 粧裕, Yagami Sayu) is Light Yagami's 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.

Sayu in the film
Sayu, portrayed by Hikari Mitsushima, is a high school student, does not go into a state of shock, and remains ignorant of Light's role as Kira, instead believing that he died in the fight against Kira. 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.