The synopsis of sociopathic and murderous serial killer Light Yagami from the Death Note franchise.
History
Becoming Kira
The notebook had been dropped to the human world by Ryuk, a Shinigami (God of Death). His motivation was to alleviate the boredom of his life. People began to notice when a disproportionate number of criminals began to die of heart attacks. The urban legend of Kira was born, a savior (or mass murderer, depending on the point of view) who would rid the world of crime and injustice.
The Investigation
The deaths of criminals eventually came to the attention of the National Police Organization. They were contacted by L the world's greatest detective. No one knows his identity or has met him face to face. He had determined that all criminals who had died were broadcast on TV. From this L determined that Kira needed a face and a name to kill. L also determined that Kira's first victim was only broadcast within the Kanto region of Japan, and that the first killing was an experiment.
Since the killings occurred most often at night and on holidays, L determined that Kira must be a student. Light was not worried about these developments, since his father, Soichiro Yagami, was a police chief and in charge of the NPA's Kira task force. Light could easily hack into his father's computer and copy his files in order to keep abreast of the investigation. When it became clear to L that Kira had connections to the police, he brought in a team of FBI agents to investigate the members of the Kira task force and their families. Agent Ray Penbar was assigned to tail Light.
Ryuk pointed Penbar out, and Light used an ingenious scheme involving the notebook and a bus jacking to get rid of all the FBI agents in Japan. This led to Penbar's fiance, Naomi Misora, to offer evidence in the Kira case, but Light got to her first, using the notebook to make her commit suicide. The task force found out about the FBI agents, and all but five left, including Chief Yagami, Matsuda, and Aizawa.
The task force was angry at L for secretly investigating them, so l decided to show himself to them, meeting at different hotel rooms each week, calling L “Ryuzaki” for security purposes. The information that Penbar's fiance committed suicide shortly after he was killed by Kira led L to investigate those who Penbar was following. As Penbar was only tailing two families, that narrowed down the suspects considerably.
L paid the most attention to Light, as he was a student, in one of the suspect families, and had a strong distaste for injustice. L had cameras planted in the Yagami house, but Light sensed that something was wrong when of his devices used to tell if someone had entered his room was out of place. He then had Ryuk, whom no one else could see, search the house for cameras and microphones. Ryuk found 64 cameras in Light's room alone. Light managed to fool the cameras by getting the names of criminals from a miniature LCD TV hidden in a bag of potato chips.
Not deterred, L had the cameras and mics removed. While taking the entrance exam for To-Oh university, Light noticed a strange examinee who crouched at his desk and went by the obviously fake name of Hideki Riyuga. At the entrance ceremony, Riyuga correctly revealed himself to be L, having decided the only way to advance the investigation was to have constant personal contact with Light. He even brought up the suggestion that Light join the task force, as he wanted to become a policeman after he graduated and had in fact assisted the police in previous investigations.
The Second Kira
Things took a turn when Sakura TV received video tapes from someone claiming to be Kira. The voice on the tape was scrambled and the video only showed "Kira" in a unique font. As several entirely innocent people died during the broadcast for no real reason, L determined that his was in fact a second Kira with similar powers to the first. Also, the incident revealed that this new Kira only needed a face to kill. Light was excepted onto the task force, in order for L to watch him more closely. The task force decided to make counter tapes, claiming to be the real Kira, begging the second one to stop killing innocent people.
Light was only too glad to go along with this, as he believed this new Kira to be a nuisance. Sure enough, the second Kira sent more tapes in response, agreeing to Kira's terms and asking to meet him. He even mentioned recognizing each others' shinigami. The second Kira also said that they had the "eyes," meaning that she had made the shinigami eye deal, where she could see a person's name and lifespan just by looking at their face. He managed to communicate a meeting in Aoyama. Light and Matsuda went to investigate, but found nothing.
The second Kira had recognized Light because you cannot see the lifespan of a Death Note user. Later that night, The second Kira went to Light's house, where she revealed herself to beMisa Amane, an upcoming model and minor celebrity who had been in love with Kira ever since he punished the criminals who had murdered her parents.
She had been given a Death Note by Rem, a female Shinigami who had fallen in love with her and only wanted Misa to be happy. Because Misa had been unable to stay away from Light, L combed over the tapes she had sent, and found physical evidence on the tapes such as her fingerprints. He then had her arrested under suspicion of being the second Kira and had her put under somewhat aggressive interrogation by his handler, Watari. Rem threatened to kill Light if he could not rescue her, so Light came up with a plan.
Imprisonment
First, Light got Rem to convince Misa to renounce ownership of her notebook, which erased her memories of ever owning and anything to do with the notebook. Then, Light accepted that one, giving his to Rem with the instructions to give it to an insanely greedy individual. Light then gave the notebook currently in his possession to Ryuk with orders to hide it. Light went to the task force to turn himself in. L put him in a cell under surveillance. After a few days, he renounced ownership with the words "get rid of it." Kira style killings then continued to occur, and L released Light after one last test. But L continued to suspect Light, and had him handcuffed to himself, so Light would always be under L's watch. Unaware of his past, Light assisted the investigation in earnest.
Yotsuba
Rem had given the other notebook to Kyosuke Higuchi, an executive at the Yotsuba corporation and a member of the Yotsuba group, which ran it. He was also a self centered man who viewed other people as tools to be manipulated. Higuchi continued killing criminals, but also killed those that would benefit Yotsuba.
The other members of the Yotsuba group noticed this and began holding meetings to decide who should die, but they remained unaware of who was doing it, just that it was one of them. The pattern of the killings came to the attention of L and the Kira task force. L managed to have cameras and bugs placed in the group's conference room. They even had Misa sign on to be Yotsuba's spokesperson, so she could meet the members of the group and make a report on each one.
Higuchi revealed to Rem that he planned on marrying Misa, like it or not, after he eliminated his competition and became rich. Rem acted in Misa's defense, and touched her with a piece of the notebook, after which she informed Misa of her past while warning her about Higuchi. Misa then approached Higuchi and managed to get him to reveal his power, which cunningly recorded. This proved a boon to the task force, but they faced a new hurdle.
The Japanese government decided it was best to leave Kira alone, and pressured the NPA to fire those who didn't quit the task force. All agreed to stay on except Aizawa, who had a wife and daughter to support. L and Light then set up a plan to capture Higuchi using other members of the Yotsuba group and a TV broadcast. The plan almost failed, but Aizawa saved the day with a well timed road block. Higuchi then revealed the existence of his notebook, and those who touched it could see Rem. It was given to L and Light, who regained his memories when he touched it and killed Higuchi with a needle, his own blood, and a piece of the notebook hidden in his watch this whole time.
L then unchained Light, but still continued to suspect him. Light retrieved his other notebook, regained contact with Ryuk, and began his plans to get rid of those who stood in his way.
L's Demise
Light instructed Misa to begin killing criminals again. This was designed to have her fall under L's suspicions once more. In order to save Misa, Rem killed Watari and then L with her notebook. Because she intervened, Rem died and was reduced to ashes due to breaking one of the laws of a Shinigami: Don't save a humans life. Her notebook was left behind, and Light was now in control of three Death Notes. Light took over as the new L, and vowed to continue the investigation, while still secretly working as Kira. However, word of L's death reached Wammy's House, the orphanage where L grew up and was training possible successors.
4 Years Later
After Light graduated from To-Oh university, he joined the NPA, which had since accepted back the task force members, making it easier for Light to act as Kira. Suddenly, a new threat reared its head. Mello, the second choice to succeed L, had joined with the Mafia and managed kidnap NPA deputy director Kitamura, asking for the captured notebook as a ransom. He did this in direct competition with Near, the first choice for successor. Whoever could solve the case L could not would succeed him and become the new L. Knowing he was inferior to Near, he turned to criminal methods to win. Light decided to make Kitamura kill himself, but Mello overcame this setback by kidnapping Light's sister Sayu.
The task force made every preparation possible, but Mello had planned things out too well and got the notebook in exchange for Sayu. Light then had Misa use her eyes to identify which Mafia man was now owner of the notebook, and made him reveal the location of the Mafia base in LA. Light then had Ryuk deliver Rem's notebook to the task force, and his father Soichiro was given use of it, and made the eye deal. In cooperation with the US special forces, Soichiro led a planned attack on Mello's base and retrieved the notebook, but was shot down before he could write down Mello's real name. Mello then blew up his base and escaped. Soichiro died soon after. The notebook was then reclaimed by its original owner, a Shinigami named Sidoh.
Battle with Near
However the task force had already contacted Near through the use of computers, who was helping them retrieve the Notebook from Mello. Near however was already susp3ecting that Kira and the Second L were the same individual so he had his voice disguised. This suspension caused some tension in the task force and eventually Aizawa came to near with information revealing Light was in fact the Second L. After a failed attack caused by Kira followers in an attempt to get rid of Near, Light had Misa give Ownership of her death not to a prosecutor and Kira follower Teru Mikami.
Kiyomi Takada
Mikami wisely choose Kiyomi Takada a former girlfriend of Light and popular news anchor to be Kira's spokesperson. Under the premise of investigating her in a bugged room without cameras they secretly communicated through notes. Through this revealed he wanted her to contact Mikami so she could receive pages from Death Note and assist him and tell Mikami to keep a fake Death Note on his person. Mello however unexpectedly kidnapped her and she killed him with a piece. Once hearing this Light had a piece of paper on his person to kill her. Unfortunately Mikami went to the safe deposit box and wrote her name on the real Death Note almost simultaneously allowing Near find out its actual location.
Final Confrontation
Near agreed to meet Light face to face at a warehouse known as the Yellow Box. Light planned for Mikami to show up after both task forces (Light's and Near's) had arrived, stand outside with the door open just a crack, and write down the names of everybody but Light. Mikami did so, and when victory seemed to be within his grasp, Light all but confessed to being Kira. When it became clear the plan had failed, Near revealed his ruse with the fake notebook. Mikami ran in weeping, and calling Light "God."
Death
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The death of Light Yagami, both disgracefully (Manga/Top) or peacefully (Anime/Bottom).
Manga
Light is left incapacitated both physically and mentally. Light yells for Mikami to write down the names of everyone present, but Mikami responds by asking how he is supposed to kill them with a fake notebook. This brings Mikami to the realization that Light is not "God". Light becomes increasingly distraught and begins asking for help from Misa, only to be told by Aizawa that she isn't here. He then asks for Takada and is told that she is dead. In complete desperation, Light begs Ryuk to help him by killing everyone in the room, yet Ryuk reminds Light that he is on neither side. Ryuk fulfills his promise from the beginning: he writes "Light Yagami" in his Death Note, and Light futilely asks Ryuk to reverse the process. He begins to panic, repeating in his head "I don't want to die", before the Death Note finally takes effect thus killing him.
Anime
As he had now lost, Light went completely mad and admitted that he was Kira. Light made a last ditch attempt to kill Near with his blood and the piece in his watch, but Matsuda furiously riddled Light with bullets before being restrained. After Mikami kills himself, Light fled the warehouse and ran away. During this, Light imagined the life he would have lived had he not found the Death Note before eventually giving up in a warehouse and collapsing on the stairs. Ryuk, who didn't want to stick around while Light served a prison term, wrote Light's name in his own notebook. Light, realizing what was happening, accepeted his death and died peacefully, a sharp contrast to his death in the manga