Shinobu Sensui also known as Black Angel, was the main antagonist of the Chapter Black Saga in the anime Yu Yu Hakusho, as well as a former Spirit Detective in his youth, having been recruited by Koenma for his prodigious talents and his unequaled sense of right and wrong. Unlike his predecessor Younger Toguro, he has intelligence and strength, whereas Toguro's knowledge was only useful for fighting. He is generally polite (sarcastic politeness in his Minoru state), kind and honorable as he respects Yusuke's friend's wishes to avenge his supposed death. Shinobu also has guilt over killing others as he looks sullen after killing Yusuke and is irritated when he is forced to kill demons once again. He does regret the path he took as he blindly slew demons in the name of humanity, to which he discovered were just as cruel if not more so.
Shinobu Sensui also has multiple personalities. He developed six other personalities to carry his various burdens. He is also a master of Resshuken and he also knows how to use sacred energy which is a special type of energy that is said to be the purest and most-powerful kind, which Koenma calls the highest echelon of power. He also said that achieving the use of this type of power takes at least forty years of discipline and grueling training. But Sensui managed to cheat by dividing the forty year-training requirement amongst his other personalities, effectively managing to achieve this pinnacle in just six years. He then added that an extra requirement was that people had to be receptive to this power and possess a desire to correct the world around them, operating on a higher plane and transcending the physical realm, all of which were qualities that even Genkai did not have.
One of the most badass characters from Yu Yu Hakusho imo.
Shinobu Sensui was once a rei kai detective (spirit detective), who had amazing judgement and powers since he was a child. But on one case he came upon a group of humans torturing innocent youkai (demons). Sakyo was in this group. When seeing that, he tried to protect the youkai, but failed. After that, he gathered a group of seven people with supernatural powers like his. Sensui planned to make a hole in the barrier between Ningenki (human world) and Makai. Although the only reason he planned it was to get Yusuke's attention. For he still had other plans. Although he may not look it, Sensui's powers are very strong and he is rated as an S-class demon. He also has seven different personalities in his one body that come out in different situations. (Though you can never tell...)
Since his powers exceed Yusuke's, he kills him. Hiei, Kurama, and Kuwabara had no chance against him either. Shinobu Sensui is defeated when Yusuke is reborn because of his youkai blood and gains S-class powers. One of Sensui's great powers was his Sacred Energy technique.