Genwaku Kyoushu was an A class demon who was hired by Yoko Kurama to kill Yomi and his band of thieves, for betraying him and venturing into a fortress without Yoko's permission, potentially endangering the entire gang.
He is voiced by Jeff Johnson in the English dub.
History
Presumably, he was not allowed to reveal who sent him for his mission. He easily overtook the weak members of Yomi's team, and Yomi was the only one who could stand up to him. The rogue's butterfly knives were perfect for fighting Yomi however, and he managed to blind Yomi by slashing his eyes, after wounding him many other times. In fact, the only time he was hit was when he let his guard down after blinding Yomi. The wound he received was nowhere near fatal, but nonetheless he chose to be cautious and fled with the job half-done, saying "Life's more important than the reward."
Three Kings Saga
For about 1000 years after being blinded, Yomi became a makai king, and spent a long time searching for Genwaku to find out why he had hired the demon to attack him. Yomi was eventually able to find Genwaku, and made him his prisoner. For not revealing who sent him (out of fear he would be killed should he tell Yomi), Yomi tortured him for years. By the time Kurama was invited to Gandara, the assassin's condition had deteriorated to such a poor condition that during a later conversation with Yomi, he begged to finally be killed by Yomi. Yomi (with Kurama present) still required that he'd answer the question before he'd kill him, and when he revealed that it was Yoko, Yomi killed Genwaku out of fury, almost completely forgetting that it was his request.