Arth (アース Āsu, can also translate to Earth) and Ellie (エリーErī) are a crucial ally team introduced during the Faudo Arc. Their ability is swords. Also, they have a natural ability which is the sword Arth used can absorb other mamodo energy when they get hit or touched.
Arth is a mamodo who appears like a living armor and descended from a long line of peacekeepers. He hopes that when he becomes the Mamodo King, he can write up laws that will bring about peace and stability. He initially encounters his partner living in a Finnish hospital. But because she was a small girl living with a terminal illness, Ellie initially refuses Arth's request, believing that she would die in the midst of the battle to be king making him vulnerable. However, when Gani Fest starts attacking the defenseless Arth, Ellie finally takes up his book and sides with him in the fight. Although small and young, Ellie is wise beyond her years and is capable of great strategies while Arth knows of many of the legends of the Mamodo World. Arth and Ellie's relationship is notably similar to that between a master and servant (at Arth's request). Arth is shown to be extremely soft when concerning his partner, demonstrated when he starts crying after Ellie's curse was lifted, much to Ellie's embarrassment.
Initially encountering Zatch with knowledge of the threat of the awakening Faudo Tower, Arth and Ellie test them to discover the power of the legendary Bao spell, which they understood were key both to awaken and ultimately defeat the massive mamodo. However, Riou still uses the duo to his own advantage, placing his curse on Ellie and subsequently reawakening her disease. However, Ellie still resists to fight along Riou, who sends Hougan to call them back, but the duo is easily defeated. Due to this, Arth, as well as Zatch, play along in using their spells to wake Faudo. But after lifting the seal, Arth soon helps in the effort to eliminate the tower, working with Karudio to destroy the Heart Guardian and set the controls for return.
After Faudo's defeat, Arth and Ellie were one of the ten remaining mamodo teams in the King Festival. Because of the strain suffered during the battle of Faudo, Ellie's illness worsened. Coupled with the discovery of a cure, Arth wishes to depart for Ellie's survival, much to her dismay. Soon after, they were the first team targeted for send-back by the beetle mamodo Gorm. Because Ellie's illness gave them a disadvantage, they were ultimately defeated. As Arth fears for his possible death by the King's power, Ellie reassures him that Zatch and his allies are still in the competition and that she will plead with Zatch to allow him to live and become his legal advisor. After sharing one last hug, Arth returns to the Mamodo World feeling certain he will be revived. Arth ranked 10th place in the fight overall.
Ellie is shown praying hard for Zatch's victory against Clear after receiving their letter. Arth is one of the mamodo who appears to Zatch when his book becomes gold during the fight with Clear Note. One month after Zatch becomes king, Arth has been appointed as his legal advisor and is shown helping Zatch with his study of how to become king.