Light Yagami is the protagonist and anti-hero of Death Note. At the start of the series, he discovers the Death Note and uses it to kill criminals, then becoming known to the people of Japan as Kira (based on the Japanese pronunciation of the English word "killer"). Light is characterized as an extremely intelligent yet bored young man, confident in his own sense of logic and how clever he is in regards to others. He is frustrated by the lack of justice in the world. Believing the world to be "rotten," he desires a means by which he can enact his will to purge the world of all evil. Light is unaware that he is not alone: a Shinigami (God of Death) named Ryuk is tired of the rotten Shinigami Realm. This Rogue Shinigami drastically changes Light's life and world forever.

In the year 2003 (2006 in the anime), Light Yagami discovers a mysterious notebook on the ground at his school. Written on the cover: the words "Death Note". The Death Note's instructions state that if a human's name is written within it, that person shall die. Light is initially skeptical of the notebook's authenticity, but after experimenting with it, Light realizes that the Death Note is real. It is in this way that Light Yagami found his true purpose in life: to become Justice by judging and killing all criminals with the Death Note. Light feels that he is the only one fit to judge humanity, thus he decides to create a New World where he alone is "God." After meeting the previous owner of the Death Note, a Shinigami named Ryuk, Light seeks to control the world with Divine Rule, passing judgment on those he deems to be evil or those who get in the way of his goals.

A short time later, Light sets to work ridding the world of criminals, attracting the attention of the master detective L. Light and L encounter each other in several different battles of wit and logic throughout the first arc of the series. Once the second Kira, Misa Amane emerges, L is able to corner them both. After giving up the Death Note briefly to clear their names, Light is able to manipulate Rem into saving Misa's life. Kira's goal: to kill L, antagonist and arch-rival to Kira. When L is attempting to test the Death Note, Rem realizes that Light has trapped her. She is forced into a situation where she must choose: the life of Misa Amane, or her own existence as a Shinigami. It is only then that Rem has an epiphany: Light Yagami has surpassed all Shinigami, having the ability to even kill "immortal" Shinigami. Rem kills both L and Watari, and then dies, leaving her Death Note at Light's disposal.

With L dead, Light takes up his mantle in order to hide L's death from the world, becoming the second L while also putting him in the perfect position to continue killing criminals as Kira. He comes into contact with L's true successors, Near and Mello, who are out for revenge in the name of the true L: Mello wants Light dead, Near wants him exposed as Kira. After making a mistake in using Teru Mikami as a proxy, Near is finally able to corner Light. Having decided that it is time to end Kira's game, Ryuk decides to kill Light.

Light constantly tries to evade the suspicion of L while both maintaining his identity as "Kira"—the name derived from the Japanese pronunciation of the English word "killer" (???, Kir??)—and cleansing the world of crime. The fact that he is a genius also adds to his notion that only he is fit to judge humanity and steer it on a proper moral course.

Light Yagami, born on February 28, 1986 (1989 in the anime), is a third year high school student (12th Grade) at Daikoku Private Academy while also attending 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.