Horologue

The Horologues (計, Kei) are demons and primary antagonists of the story.

Summary
Horologues are demons that feed on humanity's time. When a person begins to feel regret and wishes to return to the past, their regrets lure out the Horologues. Initially, they might look like granting people's wishes in order to reverse time; however, what they actually do is devouring their time. As such, they cause the flow of time to reverse for the humans they target, and they ultimately revert humans to non-existence. The amount of Horologues that will appear are varies, individually based on a human's feeling of regret. In Koyuki's case, she only lured out one Horologue. On the other hand, a gambler had lured out a bunch of Horologues. Their sizes are small to begin with. However, when they devour more time, they begin to grow. A long time ago, the god of time, Chronos, stabilized the flow of time. As such, he fought the Horologues and managed to put an end to the war. However, a small faction of Horologues survived. Horologues are able to take on human forms.

Trivia

 * Horologues are simply called "Kei" in the original version, which is derived from "Tokei" (時計), Japanese for "clock."
 * In the anime, they bear similarities to that of a Black Ghost from Ajin. Despite this, Horologues are bigger in terms of size.