Yopopo (ヨポポ, Yopopo) and Djem (ジェム, Jem) are a team Zatch and Kiyo encounter during their British adventures. Their abilities are the usage of music. After discovering Yopopo, Jem decides to adopt him as a little brother of her own. However, she soon gets sick of him due to his inability to speak Earth language (talking through his own name over and over or singing a pattern) and his desire to take part in the Mamodo fight, which Jem wanted to avoid at all costs. With his bookkeeper avoiding him, Yopopo spent most his time alone, singing and dancing in preparation for battle.
At first, Zatch was attracted to Yopopo's dance but Jem attacks Zatch. Then, when Jem was about to say a spell, Kiyo stops her. Jem asked Kiyo and Zatch if they would like to have a picnic with her. She gave Yopopo his lunch, but when he tried to sit with them she looks at him coldly and responds by yelling at him, "Don't open it over here!! Go over there!! Didn't you understand when I said I hate you!!!" Then with a look of hurt, Yopopo went to a spot by himself. Kiyo then states that Jem called Zatch a mamodo and Jem tells him about mamodos (not knowing that Zatch is in fact a mamodo). She then says her and Yopopo are looking for one for revenge. She starts telling the story of when they first met. At first, Yopopo came to there house because he was lost, and Jem's parents took him in and gave him food. But when her mom tried to ask him what his parents' names were, all he could say was Yopopo, and because of that, her parents thought that he couldn't speak their language, but Jem happily points out that he is saying his name (hence the name Yopopo). Jem finds Yopopo's book and asks if this is his which he responds by reaching for the book saying "nopopo". But Jem smiles and tells him to relax and that she can read the word for him. When she does so, she says the first spell causing a flash of light behind her and demolishing the ground in front of Yopopo. Jem suprised at first, then smiles and tells Yopopo that she can do magic now (not knowing that it was Yopopo who did it).
Zatch praises Yopopo for how long he had been dancing and asks Jem if he had been dancing all morning. Jem says without blinking that he's been dancing like that for a month straight. Zatch is shocked, and asks Jem why he would be dancing that long, and she tells him Yopopo is trying to attract the mamodo's and get back at them for what they did to her parents and grandpa, which is why Jem says he hates Yopopo, and she starts crying.
When a mamodo finally finds Yopopo, Yopopo tries to fight them without spells and he gets badly hurt. When the new mamodo was about to finish Yopopo, Zatch shot a Zaker and stopped the mamodo's assault. Jem says her spell and Yopopo knocks the mamodo into the water. The momodo retaliates with a spell that Yopopo knocks off course, but it ends up hitting Yopopo's book. Seeing Yopopo's book on fire, Kiyo is then able to summon the fourth spell Bao Zakeruga, and he ends up breaking the mamodo's outer shell. Yopopo smiles and encourages her to use the spell and Yopopo defeats the mamodo. Jem tries to put out the book's flames with the water, but it doesn't work. Then Jem runs to him and cries to not leave her, but he smiles and says her name before vanishing completly, which then she says she loves him, and Yopopo smiles and keeps saying Jem's name for the first time. He then disappears, Jem is then left there crying and saying Yopopo.
During the fight with Clear Note, Yopopo appears to help Zatch by distracting Clear by using his spell, Shin Yopopoi Topopoi Supopopoi. In the mamodo world, he was shown dancing along with other mamodo.