There are many theories people have come up with about Urotsuki, such as possibly suffering from manic-depressive syndrome (bipolar) or another personality disorder. Different portrayals of her in the game show that it is likely she changes personalities quickly towards other beings.
Neglect
It seems that Urotsuki was neglected as a child. The fact that the Child effect is rather hard to get implies this. Another event that may prove that is the child event at the Underwater Amusement Park, where Urotsuki is shown to be alone while other people pass, and the light is focused on her. Maybe this is based on a true story, and the spotlight on Urotsuki represents her feeling closed in and lost? Another piece of evidence that may prove this is a girl similar in appearance to Urotsuki in the Christmas World: If you notice, her shirt and skirt colors are inverted. She sits on the bed in the house all alone. Her room is also bigger than Madotsuki's. Perhaps she has rich parents, but is neglected by them.
Transgender
Unlike Yume Nikki and other fangames such as .flow, there are two effects for Urotsuki that allow her to change genders (Boy and School Boy). Her haircut is much more gender-ambiguous than Madotsuki's was. Keep in mind that the only bathroom in the game is a boy's bathroom, which she can get a wallpaper from only if she uses the Boy effect. The only feminine effects she has are Maiko, Twintails and Red Riding Hood. Also, it may be noted that there are significantly more boys than girls in her dreams.
Reverse Order
What is thought to be a dream is reality, and vice versa. Urotsuki's wallpapers and TV in her "dream world" do not change, but in "reality" she gains them by interacting with certain things in her "dreams." The wallpapers in her dreams are memories from reality. In reality, she is from an odd kind of universe where death is considered sleep, and creatures come back. Reproduction is unneeded, since the world expands at select points in time. Urotsuki has some interest in morbid creatures and objects, and has strange wallpapers and dreams of suicide at a certain point, and takes close looks at disturbing things and murders (or rather, puts to sleep) other beings. The newest ending (Ending ?) also supports this theory. The ending shows Urotsuki jumping from the balcony, then a puddle of blood. The only way she could've died was if the dream world was the real one, since every other "death" simply "wakes" Urotsuki up.
Strict parents
There is an ending that gives a hint about this. As you may have seen, Urotsuki only gets out of her apartment in her dreams, this might suggest that she ran away from her home and parents and is afraid of what may happen if she shows out there again (taking into account the vivid escape-like nature of her dreams). It is like she wanted to build her own world with her own rules. Every dream world related to the family, studies or work is usually dark and/or monotonous (for example: the School is filled with chasers). As you can see in Ending #2, after she leaves behind some things related to her dreams, which she considered to make her powerful (effects) and realizes about her mistakes, so she tries to make up for that going back home.
Most of the dreams aren't actually hers
Unlike Yume Nikki, most of Urotsuki's dreams don't shed any light on an overarching story. They're all random with absolutely nothing to do with each other. Well, this is because they aren't originally hers. Unlike Madotsuki and most fangame protagonists, Urotsuki only lucid dreams for fun, not as a coping mechanism for trauma, and isn't actually a shut-in/hikikomori, which is backed up by the ending/endings in which she goes outside. As such, the only dreams that are hers are the ones in the Nexus; the rest are other people's dreams, all part of an entire dream world. This is paralleled by the fact that Yume 2kki is developed by many different artists and developers. Most of the NPCs are just other dreamers using effects, and a lot are there for the same reason as Urotsuki - to have fun.
- The Beret Sisters are the daughters of a train conductor.
- Seishonen is Poniko's brother.
- Odorika is a dancer who uses her effects to create even more interesting dances in the dream world. She lost one of her eyes in the real world due to an accident during a performance.
- Kamen Eshi is a professional artist.
- Aoshiru is a boy with a terminal illness.
- The boy in the cap from the glitch ending is connected to Urotsuki in the real world, either as a friend, a relative, or a lover.
And the list goes on.