Shinobu Sensui

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.