Shuei

He is a a wandering monk who has been exterminating yokai in Saiyuki chapter 10-14.

Background
Shuei, is a master known for his power in using charms or fudo in Kinzan Temple. He was close to Kouryuu, defending the teen from the scorn from other acolytes. On the night Kouryuu became a Sanzo priest, Kouryuu gave Shuei a bead necklace he wore when he was found by Koumyou, making Shuei the very first person Kouryuu gave an item to. Then, Kouryuu went to Koumyou Sanzo and received his title, right before Koumyou was killed by a group of youkai.

After Kouryuu went on travel to seek his master's stolen sutra, the group of youkais went back and brought calamity to the temple. Shocked, distressed, and angry for their weakness, Shuei cursed himself with the forbidden charm, the curse of Araya, which gave him the powerful pure strength of Buddha to destroy youkais. He then traveled to destroy youkais and later he was known as Rikudo, a dead body without a soul.

Rikudo Arc
One night, ten years after cursing himself, Rikudo exterminate a youkai that was stealing and eating food in a kitchen in an inn. That's where he came across Sanzo-ikkou. Recognizing Goku, Gojyo, and Hakkai as youkais, he was about to kill them before Sanzo stepped in and call him Shuei. Rikudo was surprised to see the monk all grown up, but then he proceeded to have a match with the youkais outside the inn. He tried to kill Goku, but stabbed Sanzo instead in the process. Goku went berserk at the sight and bite him on the shoulder. Rikudo's bead necklace, the one he received from Kouryuu, flashed a light, making the heretic back away. Rikudo took the chance and ran off.

The next day, Rikudo's bead necklace resonated to a voice. He pursued the caller and found Sanzo meditating in the middle of the forest. Sanzo told him that he was going to kill Rikudo with his own hands. Rikudo accepted the challenge and used a movement binding spell on Sanzo. He sat on Sanzo and was about to choke him death, before Sanzo made him aware of his own weakness. Rikudo got some sense of his self back and suddenly the bead necklace broke off, making him totally possessed with the cursed charm.

Rikudo, or probably Shuei, held Sanzo's gun to his own forehead, pleading the young monk to end his life. Sanzo pulled the trigger.