Poniko
Poniko is a character appearing in Madotsuki's dream. She is held in high regard with fans.
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. Since it is unknown whether or not Poniko is actually Uboa, the light switch does not count.
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 easily 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 low-pitched "aaaaaaah" sound (if properly filtered), surprising most people.
Once the event happens, you cannot go out the door. If you touch him, you will be sent to a dead-end room with white water, red sky and a strange monster in the background, and Uboa's face will change.
Fan Speculation
Hopelessness - Due to the aspect of being trapped, Uboa could be interpreted as a feeling of hopelessness.
Appeal - This secret may be the greatest appeal for downloading the game due to spread of Uboa to non-Yume Nikki websites.
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 Uboa dancing in the foreground. You will be reminded of Stupid Sexy Flanders.