The Hivemind is the main antagonistic force in Boogiepop in the Mirror: Pandora. They are a product of a large scale infection of a specially manufactured drug, created by the same organization responsible for Kit's creation.
Like Kit, this drug is a biological weapon, designed to transform the person subjected to it into a mindless 'zombie' following a single order: to kill the first person it sees.
After Kit was rescued by Nagi Kirima from the grasp of the organization that created her, one of the men working for said organization betrayed his comrades and ran away with the drug. However, while making his escape, he was attacked and killed by a group of thugs, who, unaware of the true nature of the drug, consumed it. Having come under its influence, the 'zombies' found their target when they saw a picture of Kit that the man was carrying.
The 'zombies' can display a superhuman amount of strength and dexterity, and are capable of acting in complete coordination in order to achieve their goal. Furthermore, they can spread the effect of the drug to anyone they bite, allowing them to advance from a small group of thugs to an entire army.