A wallpaper featuring Urotsuki smiling.
Prowl (Possibly from part of Japanese verb うろつき回る, to wander, to roam)
Unknown, likely around Madotsuki's
Urotsuki wears a purple plaid shirt with what looks to be a short white skirt and red shoes. Her hair is a blonde-ish light brown, short, but with longer strands coming down from either side. From her appearance, it's likely she's in her teens, no older than 18, though arguments could be made for any age. Like Madotsuki, her eyes are always closed, but wallpapers which picture them open illustrate them as mostly lavender, sometimes brown or red (often in wallpapers where death/blood/murder is involved). Her sprite maintains the typical head-to-body chibi ratio of Madotsuki and many fangame protagonists like her.
Like the protagonist from the previous game, Madotsuki, Urotsuki appears to be a hikikomori, refusing to leave her room; If you interact with the door, she'll shake her head in a number of ways (Unless you have the Key, in which case you can enter the Developer Room through the door. As this is an out-of-context bonus area, it changes nothing). Her room is noticeably larger and more furnished than Madotsuki's, consisting of a TV, a game console, and a Computer. The view from her balcony shows a pair of peculiarly shaped mountains surrounded by a large forest, suggesting she lives somewhere quite rural.
Warning. Some spoilers follow.
Unlike Madotsuki, Urotsuki's dreamscape has a relatively lighter atmosphere. While it does contain darker areas, the majority of them are of a more neutral nature -- e.g. Geometry World, Realistic Beach, Highway, etc. Despite this, Urotsuki may have somewhat sociopathic tendencies or a strong interest in violence. When the player encounters Aoshiru, for example, her first impulse is to use the chainsaw and kill him -- the player is not given a choice.
Possible spoilers end here.
There is little to no information about Urotsuki and her relationships with other characters, but it seems she knows (or knew) many people due to the large amount of humanoid characters in the game. There has been no hint of any relatives in the game.
All theories are merely speculation based on elements of the game, the interpretation of Urotsuki and the game itself is entirely up to the player.
There are many theories people have come up with about her, such as she possibly suffers 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.
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.
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). Although there are multiple bathrooms in the game, Urotsuki only ever seems to be able to use any of them with the Boy effect equipped, one of which granting you a wallpaper upon doing so. The only feminine effects she has are Maiko, Twintails and Red Riding Hood.
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. Since this "death" simply "wakes" Urotsuki up, it falls in line with the concept of death in the real world being likened to sleep.
There is an ending that gives a hint about this. 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. 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.