Uboa (ウボァ) is a strange creature, usually considered male or genderless. Contrary to popular belief, Uboa may actually be female due to its eventual replacement of Poniko's sprite. There is a chance that it will replace Poniko when you turn off the light, and plays a continuous filtered "Aah" sound. When the event happens a bright flash fills the room, similar to a blowing light bulb, then the entire room changes and Uboa replaces Poniko (if you kill her before triggering the event, Uboa is not going to appear. However, you will be locked in Poniko's room). You cannot leave the room, and flipping the light switch only causes another flash of light. If you touch Uboa, you will be sent to a Monochrome Street highly similar to the White Desert (with even its soundtrack playing); which is a looped area flooded with white liquid (potentially a swamp or marsh), a strange monster in the background bleeding, and Uboa's face will become strangely warped. Despite this location's atmosphere being identical to the White Desert's, there are no routes that lead away from this area and into the White Desert itself. The only way to escape Uboa's Trap World is by pinching Madotsuki awake or by using the Medamaude effect.
Reception
Uboa is likely a major factor for the success of the game, especially in the English version. Many non-Japanese players were introduced to Uboa through YouTube. Uboa itself may very well be the number one inspiration to download and explore Yume Nikki by most fans.
Fan Speculation
Hopelessness - Due to the aspect of being trapped, Uboa could be interpreted as a feeling of hopelessness.
Poniko's True Form - A common interpretation is that Uboa is Poniko's true form, mostly because it replaces her sprite.
Frustration - A theory based on the correlation between the game Mischief Makers and Uboa's face, Uboa may be based on Kikiyama's and/or Madotsuki's frustration. Uboa is thematically similar to near-hopeless gameplay since it is very hard to trigger the event and even harder to accidentally discover. This may be Kikiyama's way of projecting game frustration.
Revenge - Since Poniko could be interpreted as Madotsuki's emotional side, which she is locking away, bothering her (switching the lights on and off) causes Uboa to appear, giving a fright to the player.
Molestation - The white liquid that fills the Monochrome Street/Uboa's Trap World may in fact be semen. The hills (which may also be covered in semen) that the Bloody Touching Monster is groping could also potentially represent breasts or the body in general. It appears to be more likely however that KyuuKyuu-kun is a metaphor for this potential theme in Madotsuki's past.
Madotsuki's Abusive Mother - With the manga appearance of Uboa, it is possible that Poniko is Madotsuki's mother and Uboa is supposed to represent Poniko's abusive side.
Trivia
This is lost in English, but the syllables that make up Uboa's name (ウボァ) do not normally go together in Japanese, with the effect of the name being similar to Cthulhu or Astfgl in its "wrongness".When the Uboa event activates, some of the items in the room change. The dresser gets a "face"
Dogboa (Yume 2kki) with googly eyes that may remind players of those of Masada and what seems to be a creepy smile. The two pictures change, the one over the desk turns from what looks like a mouse to a face which has googly eyes as well, and what could be either a moustache or a creepy-looking smile, and the one over Poniko's bed that changes from one shape to another. The window gets eyes and the rug's design also changes, showing a pattern with eyes as well. The lamp on the desk turns on, showing a light beam.Uboa has been one of the most popular characters from the game. Uboa has also made cameos or inspired for other characters in some fangames.
Uboa's marking (.flow)Yume 2kki - In the Sky Kingdom world there is a dog-like character that resembles Uboa. This character has been called "Dogboa" by fans..flow - Uboa makes a cameo as a marking in the dead end room in the Underwater Temple.
Mask Effect (Me)
Me - the Mask effect is a clear reference to Uboa's face, and the location it's found in is quite similar to Poniko's room.