Light Yagami thought his history was not very complicated. The death of his father, followed by the adoption of his cousin Sayu into their small family, and finally his mother's death. Many found it to be much more complicated than this.
For one, his father had been a high-ranking police officer who had been murdered on the job before Light had been born. He didn't find this to be a very important part of what made him Light Yagami, but his mandated therapist seemed to disagree. Then when his aunt and her husband had died in a car accident caused by a drunk driver who showed no remorse for his actions, Sayu had entered his life. She was what he liked to think of as the best accident that had ever happened to him, despite the mournful circumstances. This became especially true as they grew together, attached to the hip despite their age difference.
Between that and his mother's death was a bit blurry, but it mostly consisted of a stalker who had tried to kill him and failed utterly, then a happenstance with a mysterious notebook. During this time, Kira would rise as the God of the new world at the same time that Light would rise in fame. His mother was mostly to thank for that, being an amazing songwriter only seemed to help her son's singing career and together they would find love in their music. Her soft, melodious lyrics sung by a voice that almost seemed to wrench a person's heart in every which way made the duo a force to be reckoned with. Light had never had a problem talking about his mother with his friends. He seemed almost too fond of her for a teenage boy. He never minded the taunts of 'Mama's Boy' by his peers because he was proud of the fact that he was Sachiko's son.
She taught him about songwriting, but he was never able to create the soft melodies his mother hoped for, his own beat being far too fast, far too dark, and much more abrasive. Often his songs were about death and insanity, which his therapist scrutinized until there was nothing left to say about it.
Of course his therapist knew nothing about his alter-ego. It wasn't often that a Shinigami came knocking on your door, but when one did, you always answer. And of course he did. Not knowingly, but soon he came to realize his new position in the world. And only then did he decide to pave a path toward justice. If his therapist knew about it, she'd have a field day, explaining for hours how his obvious need for justice sprung from the deaths of his father and aunt and even later, his mother. He disagreed.
He saw the gaps in the justice system and felt a need to fill them. That was all. And it seemed the only justifiable reason to ever kill someone.
Then his mother was killed.
That was when his life flipped on its head. The week after her death he killed twice as many people as he had killed in the months since he first received the Death Note. Ryuk didn't seem to mind.
She was murdered by some sociopath who thought it would be fun to rape a single mother and drown her in the pool her son and daughter had gone to every summer as kids, then dump her body in the backyard for her son to find in the morning when he went to take out the trash. Her soaked body dressed only in a white nightgown, her damp hair frizzing in the humid weather, and her eyes wide open in horror. His terrified screams soon turned to uncontrollable sobbing as Sayu called the police. He didn't bother asking for his sister to call a hospital, knowing their mother, by blood or not, was beyond hope. Sayu's tears came much quieter as she attempted to console her brother.
Only a month after, he tried to take his life.
Sayu cried as she attempted to keep his blood from spilling all over the bathroom floor. He felt guilty for burdening her like this, especially after hearing her scream to wakefulness every night after and seeing the bloodshot eyes she wore since their mother's death. He spent a month in the hospital between the psych ward and healing the deep wounds on his arms. Sayu visited every day. So did his therapist.
What made it worse was how his newfound fame had made this event known to almost everyone in Japan. When he got back to school, he was greeted with silence and pity. He ignored the painful quiet and wrote name by name and researched day by day, trying to find the person who had burned his life to the ground.
He hated the people who gave him empty condolences or who acted like he was going to break from a light touch. He hated even more the ones who wrote articles about his attempted-suicide and his mother's death as if his pain were another scandal.
He tried singing a cover of Shinitai-chan and his producers went crazy over it, asking him to do covers of other vocaloids. Especially after fans and journalists became excited over the so-called return of Light's voice. He sung Clean Freak, Lost One's Weeping, and Lost Time Memory (the girl at the end of the music video reminded him of his mother) before moving on to a couple of English songs. He personally liked OCD and Panic Room. It wasn't much, but it kept people's attention until he was able to piece himself back together and formulate an ingenious plan to make the person who killed Sachiko suffer. Of course he had happened to find the perpetrator by this time.
That's when he started writing Killer In the Mirror. It was to become the name of its own album, which would contain that song and the last song his mother had written. His producers weren't very happy to be making an album with only two songs on it, but agreed nonetheless after he had proclaimed that he had more songs in the making already (a little white lie never hurt anybody).
Sayu was overjoyed to see her big brother getting back to himself bit by bit. This made him speed up the plan, wanting nothing more than for his happiness to be given back to him and believing the only way this would happen was with their mother's murderer's death. And seeing how Sayu got so excited just by the fact that he was singing again gave him the push he needed to get Killer In the Mirror out so that she could be proud of him, excited for him, and happy again.
She was there with him when he recorded the last song that their mother had written, If We Have Each Other. They both cried the first time around, but Light managed to sing the whole song after a few attempts. That night they sung the song together as they danced in the living room, crying as they mourned together. They sung the story of their mother and her love for her baby boy that she would parent by herself, the story of their long-gone grandparents that loved each other more than any two people in the world, and the story of the two of them struggling together without their parents.
They danced together with 'I love you's on the tips of their tongues, toeing around the twinkling fairy lights scattered around the room from an unpacked, lonely Christmas.