Rein (レイン, Rein) and Kyle (カイル, Kairu) are a friendly team met by Zatch in southeast Asia. Their spells revolve around brute strength. One of Zatch's friends from the Mamodo World, Rein is a large and powerful Mamodo, who nonetheless does not want to fight in the battle, both due to his own reluctance to hurt people and his bookkeeper Kyle's timidity. Although typically seen as a massive, bear-like creature, Rein has the ability to transform into a human with light blue hair. Sending Zatch a letter to come and remove him from the tournament, Rein is soon set upon by Purio and his new partner, Faudo cultist Rodeaux . With the assistance of Zatch andKiyo, and some newfound confidence from Kyle, Rein succeeds in driving away his assailants, at which point Zatch honors his request by burning his book.
Rein is one of the mamodo who appears to Zatch when his book becomes gold during the battle with Clear Note.
Back in the Mamodo World, Rein would always go on rampages, making him very feared by everyone, until the day he accidently slipped off a cliff near his village. Waiting for death, another mamodo named Zatch was the only one who didn't fear him. After that he knew what true strength was. He, along with Zatch, was chosen to participate in the battle to decide the new king. He met a cowardly boy named Kyle, who he found being shot at by villagers and became his partner. He learned that Kyle was the son of a very rich man who died and left his inheritance to Kyle; however, he was being taken advantage of by one of his father's old employees, Jill. Kyle had confidence issues, and could therefore not stand up to Jill. Due to Kyle's small stature and because of Zatch's amnesia, Zatch and Kiyo thought that Kyle was Rein the first time they saw him. It was easy to believe because so many Mamono appear to be small children. Later on, he asked Zatch to burn his book regardless if he becomes king or not because of Kyle's cowardice, but he and Kiyo decide to help him instead. Regardless, Kyle was caught by two other evil mamodo, Purio and Rodeaux. Seeing Zatch's strength, Rein was able to show his true form in front of Kyle to get rid of his cowardice and become the same as Zatch. During the battle Kyle found his inner strength and was able to defeat Rodeaux, but was unable to burn his book. Kyle passed out afterwards, but Rein still wanted his book to be burned by Zatch telling him to become King. Rein lent Zatch his strongest spell during the battle with Clear Note, and makes his last appearance helping Ted and Cherish with rebuilding the orphanage house.
Kyle was a boy who was scared of everything and had to live in a hut all by himself because of his property manager Jill. He found a strange "monster" named Rein and decided to care for his wounds. They became best friends, but as time went by, Kyle's fears become more and more great, even when Rein helped him against Jill. But eventually, when Zatch came to town to help, he was still the same until he saw Rein's true form. After Rein's book was burned he gained the courage to stand up to Jill and was able to stand on his own but even if Rein isn't there he'll ask other people for help if his power not enough. Kyle was last seen dancing upon seeing that he got Rein's letter from the Mamodo World.
Rein and Kyle are another mamodo team who Zatch got confused by. He thought Kyle was a mamodo because of his unusual nose, shaped like a bear nose and Rein was the partner because of his human form, like how Kiyo thought Eshros was the partner because he looks normal and Shin was the mamodo due to the lines under his eyes.