Ui is the third world to be revealed in Kokiro. It is where Kokiro first encounters Kit Farren.

Ui appears to have reached a similar level of technological advancement to present-day Earth, complete with television, telephones, motor vehicles and the Internet. While it has been settled by humans and other creatures, it, unlike Nia, still contains rural and wooded areas. It has been suggested that there is a problem in some areas of Ui with underground radiation, which, while not powerful enough to disrupt life on the surface, is dangerous to anyone exploring subterranean areas such as the basement of Feel Better.

Some of Ui's culture is shared with Earth, while other aspects are more original. It is unclear what the status of religion is on Ui, though Saint Nicholas, a figure from the Christian canon, has been mentioned in passing and cast extras have been drawn wearing what appear to be cross-shaped necklaces. Although Ui's other-than-recent history has not been properly discussed, there has been a single reference to the "Old Ones", possibly an older civilisation that settled in the area where Kokiro takes place.

Ui is home to a variety of sentient creatures, including humans, tailimins, Jitai, dragons, Kujian, and the legendary Kokirins. However, humans are the most prominent presence in urban areas, and some species, such as the tailimins, are unaccustomed to entering cities at all. The area surrounding Coyote is currently suffering the effects of a war involving its humans and the Jitai. It is unclear whether any active combat is currently going on, but public sentiment against the Jitai species is still generally strong, and has made its way into popular culture and the media. As a result, there is little or no stigma attached to hurting or destroying Jitai.