Сенсуй Шінобу

Shinobu Sensui was Earth's Spirit Detective before Yusuke Urameshi. According to Koenma, he was the natural choice; he was gifted with amazing spirit awareness, strength, and a strong moral sense of right and wrong. Sensui was very clear-cut and adamant about his moral codes. Humans were victims, so he had to protect them. Demons were evil; he had to kill them. In time he met Itsuki, who asks Sensui give him another day to live first so he can watch the finale of his favourite TV show. Hearing something so human coming from a demon, Sensui laughs and spares Itsuki. By hanging around Itsuki, Sensui realized that not all demons were evil.

However, his attitude towards humanity didn't change until one fateful night. Koenma had ordered Sensui to seal a tunnel connecting Demon World and Human World. This portal was being maintained by the Black Book Club (Sakyo amongst them), for use in luring demons through; kidnapping them, and then selling them as bodyguards. Sensui was able to catch the club by surprise, but in doing so witnessed a horrifying sight. He saw something known as the Feast of Human Vices, where the demons were cruelly tortured by humans, who then bathed in their blood for no purpose other than a sense of power and twisted enjoyment.

Sensui’s black-and-white beliefs about humans and demons, that one was wholly good and the other wholly evil, suddenly became very grey. Unable to handle this discovery and the knowledge of his own acts, he was driven insane. In a rage, Sensui slaughtered everyone there, and began to question the sanctity of the human race. He became obsessed with the Chapter Black video tape, which documented the worst cruelties of the human race against demons and each other. The tape and Sensui eventually disappeared. Sensui decided to punish humanity for their sins by opening a stable tunnel to Demon World, in turn keeping Team Toguro owner and (ironically) Black Black Club member Sakyo's plan alive.