Zombies are creatures in the series created by the effects of the residual magic left in Hole by Magic Users, thanks to the toxins in the magic smoke, the ground reacts and once a year during the Day of the Living Dead the corpses of people killed by magic comes back to live with hunger for flesh. The Hole Neighborhood Council is in charge of keep a track of these creatures once they are killed again.
Thanks to the constant attack of Magic Users and the increasing dead rate of humans thanks to their magic, human corpses were labeled with numbered plates before buried, as the possibility they could resurrect and the family of the deceased have to reclaim their bodies to be buried once again. During the time Ai Coleman was alive, zombies were a hidden concept to the public and the city government tried to cover, years later they could no longer hide the fact that near the end of the year the people killed by sorcerers came back as rotten magic monsters.
The resurrection of human corpses became a morbid celebration called "The Day of the living dead", where the residents of the city gathered in the center of Hole near the Department Store to prepare themselves to repel and kill these creatures, in order to collect their plates and change them for prices, like a meat mincer Nikaido wanted to make gyozas more easily. The event goes for the whole night and many business come to aid the celebration, like food and weapons stands.
Magic User corpses can also become zombies but not by magic, Prof. Kasukabe turned the slaughtered remains of Matsumura into a remote controlled Frankenstein-like zombie to play as the 3rd base in The Hospital baseball team "The Worms" using his research about sorcerers anatomy and technology, he needed to recharge every half hour or he would go berserk.
Later in the series is revealed that, even if is true that the zombies are the result of the residual magical smoke left by sorcerers, the source of the phenomena is the sinister entity from the lake of refuse, the same that took possession of Ai Coleman and the mind behind Kai actions, Hole.
During the events of the central department store all the humans inside the building were killed by the tubes from Artificial Devil Kai and turned into zombies, showing sings of possession with Kai's evil expression, having tubes attached to random parts of their bodies and fighting the remaining sorcerers inside the store regardless of how many times their limbs where chopped.
Half humans like Shin can also become zombies, they remain alive but with no will of their own, he was attacked and one of Devil Kai tubes pierced his leg, the liquid from the lake of refuse filled his body and Kai took possession of him, and started to kill or disembody every magic user in the store. He was repelled and transformed into mushrooms by En when he came back to life, the remains of Shin were cleaned with no traces of Kai influence nor the liquid from the lake of refuse, freeing him from Hole's control once En turned him back to normal.
The final purpose of these zombies is pour the liquid from the lake of refuse directly into the scorers world to recreate the effects of Hole's rain and exterminate the Magic User race once and for all, they came from a gigantic magic door connected with the bottom of the lake of refuse floating adobe the clouds of the Magic User World. Kirion was able to reach the sky in a magic broom to find the creatures with tubes coming out of their mouths pouring the liquid directly into the clouds, they seems to be even mutated, having multiple heads and an advance decomposed state.