Poniko
Poniko (ポニ子, "ponytail child") is a character appearing in Madotsuki's dream. She is held in high regard with fans. Her house is found by diving into a pool located in an igloo at the Snow World.
Description
Poniko is a blonde girl with a ponytail, green shirt, a skirt and red shoes. She, like Madotsuki herself, lives alone in a very bare house. She seems to want to be alone, as Madotsuki is only able to reach her house by going through a Pastel Lake. Poniko changes in a random event triggered by turning off the light in her house.
Fan Speculation
Room - Poniko and her room could be interpreted as early designs of a protagonist and her room, due to familiar aspects seen in Madotsuki's room.
Actions - No matter what you do, she doesn't seem to pay any attention to you, which is the point of some fandom hilarity. The only exception to this is running at her with a knife and using the cat effect. This is a trait shared by many characters, however.
Uboa
If you turn off the light, there is a 1 in 64 chance that you will see Uboa. It was originally thought you had to kill Poniko first and return for the secret, but has been proven false.
Description
Uboa is a strange creature, usually considered male or genderless. He replaces Poniko soon as you turn off the light, and plays a continuous muted "aaaaaaah" sound, (if properly filtered,) surprising most people.
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, there is still a chance he might appear). You cannot go out the door, and flipping the light switch only causes another flash of light. If you touch him, you will be sent to a Monochrome Street highly similar to the White Desert (with even its soundtrack playing); a looped room with white water, a strange monster in the background bleeding, and Uboa's face will change.
Uboa's name originated from a type of scream in Japan. Originally pronounced 'uhoa' (ooo-hwha), when said in a manner resembling a scream, the name certainly suits its appearance and situation.
The origin of Uboa's (undistorted) face design may have come from a 1997 Treasure game called Mischief Makers, if it is not a highly odd coincidence. The denizens of the planet Clancer, as well as most objects on the planet, all have black, hollow looking eyes like Uboa that may be both curved inward or one curving outward and a usually gaping black mouth. The faces of most people are also white.
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 he replaces her sprite.
Frustration - A theory based on the correlation between 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.
Memetic Mutation
Cho Uboa - In which Uboa is a horrifying muscular creature. Poniko keeps him as her pet.
ah not tan neet... - In which Microsoft Sam sings the Toriningen Party Song, with a naked Masada with Uboa for a head dancing in the foreground. This is a remake of a Japanese video with a Japanese man singing, and the lyrics of the memetic version are mondegreens of that one.