Let's talk about Uboa:

When you get to Poniko's house in the Pink World, there's a 1/64 chance that Poniko will change into Uboa after flicking on and off the lights. But what does this huge monochrome monster mean?

I found some theories in Uboa-chan, but I'm going to copy-paste all of them, as you can imagine. Anyway, I have my own theories.

 - Notice that Poniko acts like she doesn't even know that Madotsuki is in her house. No matter what you do, or how many effects you use on her. Then you start thinking that flicking the lights on and off would be a nice idea to bother her, and yes it is!! Some kind of monster appears when you turn off the lights. In this case, Uboa could be the final form of Poniko, showing how bothered she is when Madotsuki's flicking the lights on and off. 

 - Poniko could have been Madotsuki's best friend, but our brown-haired girl did something terrible to her, that's why she ignores her. Turning off the lights would show Poniko's dark side; Uboa could have been the "personification" of her rage.

 - You have heard some theories about Madotsuki's past. Some of those theories point into the fact that Madotsuki was raped. Uboa could have been Madotsuki's torn face as she was being raped, and all the eyes and faces appearing in the wardrobe, the window, the bed and the pillows could represent  fear of being seen.

Well, those are some of my theories, but feel free to comment if you have something else to say!

Monochroma Nikki