Past

Sensui has a really dark past. He had incomparable Spirit Awareness and could at a very young age, destroy apparitions. He started his job when he was in High School. As stated by Koenma, Sensui's universe was cut and dry. Black an d white. Demons were evil so he had to destroy them. But then he began to ask Koenma strange questions one day like,"Are we even worth protecting?" One fateful night, Sensui and his partner Itsuki were doing a mission that would change their entire lives. The Black Black Club was an organization that captured stray demons that went past the Kekkai Barrier and sold them at enormous profit. All of it's members were going to have a meeting, and since they would all be assembled together in one place, then it would be perfect opportunity to defeat them. Sensui and Itsuki raided into the Mansion where the meeting would be held with ease. Sensui ran into the room where the meeting was to be held and saw the most horrific thing he'd ever seen. He saw something that changed him... grey. Truckloads of grey.

It was called, 'The Feast of Human Vices', a ritualistic party where privileged humans were torturing demons they had ensnared. Sensui had sworn his life to protecting his race from demons. But in the ritual, humans were enacting cruelties a thousand times worse than anything he'd seen from a demon. Sensui snapped... he couldn't believe that humans were doing things that he'd never seen even demons do. Black and white blurred; flipped upside down. His mind wasn't big enough to hold the change. He killed every human involved in that feast. He then stole a video from Spirit World. The video was called, 'Chapter Black' and it had thousands of hours of footage of humans doing henious and terrible crimes to their own people. It is said to turn a human against a human in a total of five minutes of viewing. After the Chapter Black video was stolen, both Sensui and Itsuki were never seen again...