Ever since he was very young, Sensui has had the power to see monsters. He started wondering why he was the only one that could see them, but he told himself that it was because they were evil, and that he was chosen to exterminate them. Later, he was chosen the Celestial Counsal to become a Celestial detective. He job would be to exterminate the monsters that were causing trouble on Earth. He became stronger and stronger, blinden by the illusion of eliminating what was bad.
For his last job ever, he had to stop the "Black Book Club, a group of humans that captured monsters and used them as slaves. During that mission, he came across something the was contradictory to everything he ever believed in. The humans were the ones torturing the monsters: they were the ones acting evil. Sensui killed everyone in that room and started to hate humans more and more every day. He eventually came in contact with a tape of all of the atrocities humans have done to the monsters. He than dissapeared for ten years..

When he finally comes back, he reunited seven human with special powers to help him open the passage between darkness and this world. He plans to let class A ans S monsters (extremely strong monsters) into the real world, for them to kill all of the humans. You must note that for Sensui, only humans are bad. He likes the plants and the animals, but hates all humans. The creation of the passage is also there for him to be able to see the darkness. Indeed, the already existing passage only allows weak monster to pass, and he has become to strong to go trough.

Sensui has developped seven different personalities through his fighting. Sensui, his main one, is the strongest, fastess and the one with the most energy. He appears to laugh a lot in this form. He has 3 personalities he uses to fight and the three others for other situation. In his other personalities, there is Minor, used for fighting, who likes to praise himself. There is also Kazuya, also used for the fighting, who can kill anyone with no regret. One of is personalities is Nal, a woman that only shows herself to his friend, Itsuko. The other personalities are uknown to use because he never gets to use them. No one has ever touch Sensui during a battle. His body is full of scars because of his training.
I could name you all of his attackes but they all have complicated names that sound alike so I'll just describe them to you instead.
One of his attack consists of materialising a ball of energy and throwing it at his oponnent. He can throw it anytime he wants, and even if he is attacked before he does, the ball stays like if nothig happened. He can also create many of these balls, and throw them all at his oponnent.
Sensui can also use his energy to fly.
He can also create an armor with his energy. He has two type of armor: one for defense and one for offense.