History:
Sato Akira has been able to see things that others can't as far back as he can remember. Since he was an Abel (or part of Abel's soul/essence/being/what have you), he had been the first innocent human touched by Death, and the first martyr.
He spent the precious time that he could get on the nearby playground in the sandbox, because there was a small boy who would be there whenever he showed up. No one else seemed to notice him, and the boy didn't talk, but Akira played with him anyway (though "playing" seemed to be more "Akira talked and the boy listened").
When Akira was five, there was a fire in his cousin's apartment. Naoya was thankfully safe, but Naoya's parents had both been trapped in their room by a jammed door and hadn't managed to make it out in time. Naoya came to live with Akira and his parents, which was fine by Akira, since he liked Naoya. It was around this time that he started to realize he could pick up on the feelings of others.
Soon after he started primary school, he met a girl named Yuzu, who at first thought he was weird since he seemed to know what she was thinking. After a while, though, she took pity on him (no one ever wanted to sit near him or talk to him), and the two became fast friends. He did tell her about seeing people who weren't there, and how he could feel the emotions of others, which weirded her out, but she could tell he was basically decent.
When he was around nine years old, he saw a man get hit by a car. He ran to the man, ducking through the legs of the paramedics. They tried to pull him back, but he hit them and kicked them and bit them until he broke through the circle of people. The man was crying, and begging something that he couldn't see to not take him yet, his daughter had just been born, he needed to see her face before he could die.
Akira watched as the man's body was taken away, and yet... the man still looked like he was there. Akira walked up to the man and asked him why he was still there. The man responded with a slightly more coherent version of what he had said before: his daughter had just been born, he was running to the hospital to see her, and what kind of father would pass on before knowing their daughter's face? Akira got the man's name, and thought about this on the walk home. After he explained the situation to Naoya (who knew a little bit about Akira's 'strangeness'), they went to the hospital. Naoya said he was there to do research for a project, and talked his way into the maternity ward. He and Akira took a few pictures, and Akira took him to go find the man again. He showed the man the pictures, and the man seemed to fade after thanking him.
Thus began Akira's self-imposed task of helping the ghosts of Tokyo.