Idiom (イドム Idomu?)(abbreviation for Idea-Monster) are monsters born from the trauma and regret of humans that manifest in the real world. They often take the form of an object or thing that is the source or a factor in the memory of that trauma that is strongest in the minds of the humans who spawn them.
Sub-Idioms are small monsters that attack civilians and try to devour them. They are weaker that the Idiom they spawn from and have a motif that relates to the object (Ex. watermelons seeds from a watermelon). They are usually dealt with by AIT's ground troops while Mikazuki battles the main Idiom
- Watermelon Sub-Idioms- Man sized bipedal watermelon seeds with razor sharp teeth and one red eye.
- Door Sub-Idoms- Flying door knockers that shoot energy blasts from their mouths.
- Watermelon Idiom- An Idiom that first took on the appearance of a giant flying slice of watermelon before transforming into a plant-reptile hybrid monster. It has the ability to make whatever it steps on or near decay rapidly as well as fire breath, needle missile projectiles, sickle vines and vines with mouths that eat objects.
- Door Idiom