"Friend" takes place sometime after the ending of the Konjiki no Gash!!/Zatch Bell! manga and is set in the Mamodo World after Zatch Bell became King of the Mamodo World. The side story starts off a young poor mamodo boy named Muumu looking at the school the King goes to. The boy went back to his own with his "older sister."Meanwhile, Zatch studying the laws of the Mamodo World. Zatch, however, became bored of his work and decided to cast his lightning magic like Zakeru for fun and to pass time. Zatch was remembering the days of being with Kiyo Takamine. His legal adviser, Arth, was scared from the way his King was behaving since Arth wouldn't be able to defend himself if his King accidentally shocks him. He explains that Zatch's wand (王杖, ōtsue) acts like a "shield" to the King by preventing mamodos from casting spells within 50 meters (164.042 feet) of the user's wand while allowing the wielder to still being able to cast magic.
Zatch went to school later, being clueless to his lessons as his teacher apologizes for not teaching well. Zatch is then in a group project with Penny and with two other mamodo children with one of them acting scared that Zeno might hurt him if he does something wrong to Zatch.
After school, Zatch wanted to play with some of his best friends including Kolulu, Tia, and Schneider "Ponygon" but his friends told him that Zatch doesn't have time to play with them due to his role as King of the Mamodo World and Zatch thinks that his only true friend is his newest variation of Volcan 300 that looked like a gourd. Zatch couldn't take not being with any friends and decided to look for a new friend and as he was floating away in the sky with his mantle floating in the air, the kids laugh at the King's naked butt and his friends became embarrassed.
Zatch found the lonely, poor boy named Muumu and asks him to be his "friend." After asking what Muumu was doing, Muumu explains that he was looking for food for his "big sister." Zatch gave Muumu his yellowtail sandwich and Muumu was filled with joy.
Muumu takes Zatch to his small wooden cart-like house and gave the yellowtail sandwich to a sewed, ripped doll that he calls his "big sister." Zatch immediately knew it was a doll but he accepted them and said he'll bring more food for them the next day, saying that they are "friends."
The next day, Zatch pays Muumu a visit. However, something was different in Muumu's face. Muumu told Zatch that Muumu made a new friend. As Muumu slowly turned around, he quickly snatched the wand from Zatch's hand and gave the wand to a large mamodo named Jiiza. Zatch asked what Muumu was doing and Muumu explained that he was bribed by Jiiza with food and money and Muumu would use them to buy himself and his "sister" a new, bigger house.Jiiza then attacks Zatch. Zatch tries to defend but with Jiiza wielding his wand, Zatch is unable to cast his spells. Muumu shouts at Zatch in anger and sadness that his real sister died since Zatch didn't help save her when Muumu asked for the King everyday. His older ill sister told him that if she and Muumu and prayed to the King, the King would gift them with food. Each day, their wishes to the King were never granted since the sister said that he was "too busy" and his sister died from hunger and illness. Muumu couldn't take any more ignorance from the King and told him that Jiiza is his new "friend." However, Jiiza betrays Muumu by stealing back the money he gave Muumu, which made him shocked and more sad that he lost his "friend."
Jiiza then attacks Muumu but Zatch defends Muumu by attacking the hit of Jiiza's spell. Jiiza wonders if he could become the new King of the Mamodo World if he kills Zatch and plans on killing Zatch along with Muumu. Zatch protects Muumu and told him what being a true friend is like. Suddenly, Zatch heard Kiyo's voice shouting, "Baou Zakeruga!" and Zatch somehow cast his signature spell, bypassing the effects of his wand, and stroke Jiiza down. In the Human World, for a second, Kiyo thought he had Zatch's spellbook in his hand but it disappeared, but Kiyo was happy for Zatch.
In the end, Zatch brought his new "friend" Muumu to his school, introducing him to all of his classmates.