If you flicker the light in Poniko's Room, eventually the Uboa Event will occur, which causes the room to darken substantially. If Madotsuki moves towards the center of the room, an image of Uboa will flash on the screen before the dream ends.
Right after 2.0 Version came out, if you had the Lamp Effect and walk to Poniko's desktop after flickering the lights with said effect equipped, you could activate an alternate Uboa Event, where you could see Uboa moving around in "Uboa's Trap World". Interacting with Uboa will give you the Effect "Mask of Fear". After getting said effect, and returning to Uboa's Trap World, Uboa won't be seen any where and the only way to get out is by waking up.
Uboa appears like a black blob or shadow-like creature with a white oval covering most of their front. The oval resembles the traditional Noh theatre masks, as well as the Greek comedy/tragedy masks that depict extreme joy or sadness. However, in regards to the Greek masks, Uboa's expression seems to be a blend of both, as their left eye is squinting, while their right eye is resting. Accordingly, the left corner of the mouth is turned up, while the right corner is turned down. In YUMENIKKI -DREAM DIARY-, they feature a more ghostly, faded appearance, than that of their more solid form in the original game.