Me, Myself, and I
spoilers

At the end of volume two, it is revealed that the real personality of the main character is Nishizono Shinji. Twenty years before, Shinji lived at a small clinic in the mountains with Kobayashi Yousuke, Murata Kiyoshi, Sakurai Kotone, Amamiya Kazuhiko, and his mother Amamiya Minako, a nurse who cared for them. One night Shinji killed all but Kiyoshi, under the pretense that they were "mean" to Kiyoshi. Then he set the building on fire. Both boys escaped, and the murders were attributed to Kiyoshi. When he emerged from the fire, Shinji seemed to have already switched to Yousuke's personality. He tells the firemen his name is Yousuke and continues to use that name as his primary identity until his girlfriend's death.

The most obvious suggestion is that Shinji was overcome with emotion at these murders, whether it was exhilaration, horror, or something else. This could cause his mind to break into shards of personalities. A similar phenomenon is found in some killers, especially cannibalistic ones, who claim that the spirits of the dead are following them or talking in their head.

However, this does not seem at all like the cold, calculating Shinji who orchestrated having Kiyoshi take the blame for him. Perhaps more likely, and more chilling, is the idea that Shinji chose to do this, so that he could not give himself away. After all, if he remembered nothing, and his personality was nothing like that of a killer, who would suspect him?