Light Yagami (夜神月, Yagami Raito) is the main protagonist of the series Death Note. After discovering the Death Note, he decides to use it to rid the world of criminals. His killings are eventually labelled by people of Japan as the work of "Kira Light is characterized as hardworking, talented, and a natural genius. Light would constantly predict all of the possible scenarios that could happen, and plan a solution in advance, even in the most impromptu situations. His only faults are his deep sociopathic interior, his cynical outlook on life, his overconfidence in himself and in his belief that he is never wrong.
He is frustrated by the lack of justice in the world. Believing the world to be "rotten," he uses his Death Note as a means by which he can enact his will to rid the world of evil people. Light's main goal is to create a new world that is free of injustice and populated only with honest and kind people. Then, he will become the "god of the new world".
Starting as a anti-hero in the beggining of the series, he gradually became quite crazy, unfeeling and cold among others in what the plot progresses, likely as a result of corruption by the Death Note's god-like powers, given when he gave-up his ownership of the notebook as part of a plan to proves Light's alleged innocence to L, he temporarily recovered his original personality.
Light is decisive and is driven to achieve his ambitions, without faltering one bit in his beliefs. Despite staining his hands with the blood of the people that he's killed, this extreme action can be said as a result of his unmoving resolve.
Through use of the Death Note, Light quickly begins to develop a ruthless and malevolent nature. He becomes completely devoid of love, compassion and mercy. He views himself as the only rightful savior of mankind and that all his actions are justified, no matter how inhuman they might be. He is shown to even be willing to sacrifice his own family to further his goals.
Throughout the series, Light is constantly trying to evade the suspicion of L and other detectives while both maintaining his identity as "Kira"—the name derived from the Japanese pronunciation of the English word "killer" (キラー, Kirā?)—and cleansing the world of crime. The fact that he is intelligent also adds to his notion that only he is fit to judge humanity and steer it on a proper moral course.
Light's Reincarnation Theory
There is a popular theory that the unnamed Shinigami featured in the anime OVA, Death Note Relight 1, is the reincarnation of Light Yagami arguments to support the Reincarnation Theory are
◾He wears a coat that looks very similar to the one Light was wearing when he died which also looks to be stained with what could be blood
◾He sports a red tie around his head that's similar to the tie of Light's high school uniform
◾He carries a messenger bag, almost identical to the one Light often carried with him when he was in high school.
◾The Shinigami's mouth is shaped like Light's "Kira" smile.
◾Light also stands in the Shinigami Realm during the end of the second intro sequence- a possible foreshadowing of his reincarnation
◾The Unnamed Shinigami also sports goggles, which could allude to the fact that Light never made the deal for Shinigami Eyes
◾The way he tosses the apple to Ryuk, as Light did when alive
◾He walks with something of a limp, which could be related to the injuries Light sustained when shot several times by Matsuda
◾Ryuk himself has some vague dialogue that could be interpreted as him referring to the Unnamed Shinigami as Light: After looking at his own Death Note, Ryuk speaks to Light as if he were there; "You were not God. You were just...gone already?" This could mean Ryuk was addressing the unnamed Shinigami as Light
◾Ryuk also allegedly addresses the shinigami as Light when he asks "Wouldn't you agree, Light?" at the very end of the special
◾ The shinigami doesn't stay to hear the end of the story, which could imply he already knows how it ends.