Prequel to Itou-san. As a stand-alone, this would be incredibly creepy and unpleasant, but reading it after "Itou-san," it adds quite a bit to the story. As if "Itou-san" isn't sad enough, this backstory even adds to it...so glad that he finds some happiness with Kyousuke in the main story, even though it's short lived...
I: Episode 0