Light, born on February 28, 1986, is a third year high school student (12th Grade) at Daikoku Private Academy who also attends supplemental classes at Gamou Prep Academy in the beginning of the story. Light has a father, Soichiro Yagami, who is a member of the investigation team opposing Kira. In addition, Light has a mother, Sachiko Yagami, and a younger sister, Sayu Yagami.
In Spring 2004 Light becomes a first-year student (freshman) at To-Oh University, becoming one of two freshman representatives; L is the other freshman representative. During a tennis game with L at To-Oh, spectators reveal that Light was the junior high school tennis champion in 1999 and 2000; After his 2000 victory, Light quit tennis as he entered high school.
According to Ohba, Light sees Misa Amane, who he uses as an accomplice, as a "bad person" who killed people, so he acts emotionally cold towards her and manipulates her. Despite this, he keeps her alive even after her memories of being Kira have been removed for the second (and final) time.
When L is finally able to corner him, Light puts his master plan in practice. He temporarily gives up his Death Note to Ryuk, erasing his memories about his time as Kira from his mind. Rem, under instruction by Light to give it to someone who would use it selfishly, gives it to Kyosuke Higuchi of the Yotsuba Corporation. Now unaware of the fact that he was the original Kira, Light accepts L's offer to join the investigation team to hunt down Higuchi, known as the third Kira. Light requests the alias Light Asahi to be used for him. When Higuchi is finally captured, Light touches his Death Note, thereby regaining his memory of when he was the original Kira. He then proceeds, exactly as planned, masterfully killing Higuchi by using the Death Note paper he put in his watch before he lost his memories; and manipulating Rem, who also dies in the process, to kill both Watari and L.
Light is later nominated by L's investigation team (who are unaware that he is Kira) to act as L, due to the fact that they cannot reveal the death of the real L to the world. Light also takes over the role of L's assistant, Watari. Now able to act as both Kira and L, Light can continue killing criminals while leading the investigation team in circles. His secret identity as L is soon discovered by the Special Provisions for Kira, President David Hoope, and Mello's gang. Light joins the Japanese Police Force's Intelligence Department in April 2009, believing that no one can stop his plans. Near and Mello eventually appear before him, trying to accomplish what L could not: to expose Kira's true identity, and bring him to justice. At this point, according to Death Note 13: How to Read, Light becomes "overtaken with evil" and possessing "arrogance that came with Kira's power"; Light loses sight of his former self.
Wolk describes Light's ideal world, a "totalitarian" place "ruled by a propagandistic TV channel and an arbitrary secret executioner." Wolk said that Ohba sometimes suggests that the world is "in some ways a better, happier world than ours."
In the end, the combined efforts of Mello and Near, combined with a large miscalculation and miscommunication on the part of Mikami and Light, force Light to reveal himself as Kira. In the final clash between Near and Light at a warehouse, all is revealed and the Kira case comes to an end after six long years.
In the manga version, after completely losing his head and characteristic cool over the prospect of being arrested, and in complete desperation after his attempts to write Near's name down are thwarted by Touta Matsuda (who had previously believed in Light most ardently of everyone involved in the Kira case), Light begs Ryuk to help him. Ryuk reminds Light that he is on neither side, and fulfills his earlier promise to write Light's name in his Death Note, thus ending Light's life.