Uboa is an evil spirit that inhabits the lake world. It's purpose is unknown, but it is very obvious that it only wants to inflict as much pain and suffering as it can against Madotsuki and any others in the dream world. It is the aspect of fear, that looming evil and unexplained that will never die, and can only be ran from.
Every attempt at killing it will only result in it getting stronger, it feeds off fear and hopelessness, the emotion of giving up can often act as a gateway in which it can come through.
Simply touching him will result in you getting transformed to his dimension, however things like knives and weapons don't seem to count, luckily.
It is belived that it is one of the aspects of Madotsuki's murderer haunting her.
The closest it had ever came to death is when Oreko's machine, in trying to help Sabitsuki and Madotsuki battled it, ripping off it's limbs until only a head remained. Just before it were to crush it's head under it's foot, uboa melts into the machine, engulfing it, and causing it to explode, and then growing bigger again. It then kills Oreko by ripping him to pieces.
This results in Sabitsuki's machine effect becoming obsolete, making her condition return, and get worse. thumb|left|174px|Uboa, when angered
Uboa can change into any desired form at will. Though he can get hurt and can be temporarily stopped by exssesive amounts of damage, he gains mass the more he is. he continually grows and grows throughout the course of the story, getting harder and harder to keep away every time he finds Madotsuki. In the end he gets so large he fills the entire horizon, attacking with a swarm of writing hands