Shinobu Sensui (仙水 忍, Sensui Shinobu), also known as Black Angel, is a former Underworld Detective and the antagonist during the Sensui arc. Sensui was a prodigy aura user and demon hunter from an early age. He was recruited by Koenma as Underworld detective and worked tirelessly. His view of demons is changed when he met Itsuki, who plead for Sensui to spare him for one day so he could see the finale of his favorite television show. Itsuki ended up working as Sensui's partner, gradually widening his black and white views on justice. During a routine case Sensui came upon an early hideout of the Black Book Club, a room where humans were torturing demons, bathing in their blood, and committing other evil acts. Seeing the cruelty of those he was sworn to protect Sensui went mad and killed everyone there. He soon disappeared, taking with him the Black Chapter, a video tape kept in the vaults of the underworld containing footage of the most evil deeds of humanity. Sensui waited until a new Underworld detective was revealed before coming out of hiding, springing his plan to judge the human race. During his years hiding Sensui created six alternate personalities to deal with the stresses on his mind. They are as follows:
- Minoru - a charismatic speaker who was in control during the beginning of the Sensui arc.
- Kazuya - a sadistic killer who takes over as Yusuke gets the upper hand fighting Minoru.
- Naru - a female personality spoken of by Itsuki who writes poetry.
- Jooji - a weaponsmaster.
- Makato - in charge of miscellaneus: cooking, cleaning, gardening, shopping, etc.
- Hitoshi - looks after nonhuman things such as plants, animals, etc.