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.
