Everyone has someone who they were born to be with. Everyone in the world has a red string of fate tied from their heart to someone else's, and that's the person who they're meant to be with for the rest of their life.
Sometimes, they find that person. Sometimes, they'll grow up with the person, and sometimes, they'll meet the person for the first time moments before their death.
"Katou!" It was Kira's voice, and Katou looked up from the engine he was working on.
"Kira, hey," Katou said, looking back to the engine.
It wasn't a new feeling now when Kira said his name. The feeling of his heart tightening in his chest for just a moment. He was used to it now, and had been for a long time. He had known, from the moment he first saw Kira, that Kira was His One. He chose to stay close to Kira, to follow him, because he knew that someday Kira would realize this as well, someday.
Of course, for some people, someday never comes. As Katou stood outside of the gates of heaven, waiting for death to claim him, he knew he was one of those people.
He realized in that moment, moments before his death, that there had been some sort of mix up with those strings of fate. After all, strings got tangled and lost so easily; though he was bound to Kira, Kira was bound to someone else. Someone from long, long ago.
Though Kira, the original Kira, the Kira who had been, though his heart was affixed to Katou, the Kira who took over the young Sakuya's heart was attached to someone else. Since the original Kira had died years ago, his string had been severed.
Katou had realized this when Kira became Lucifer. Wasn't that just like him though? After all, he couldn't really do anything right, he didn't really deserve to find someone to love him.
He felt a presence beside him, and his cigarette lit. When he looked up, there was Kira standing above him. He felt his eyes prick a little, and he was sure if he had just a few more moments to live (even if he was already long dead) he would've cried.
The Red String of Fate was stronger than he gave it credit for, and though Lucifer was bound to Alexiel, Kira, no matter who he was, was bound to Katou.