Ultra Violet/theories

Sometsuki is/was Being Bullied
All of the chasers in the game look very much like schoolgirls, possibly representations of people who picked on Sometsuki at some stage.

One area in the game is a school-like library, where all but one of the NPC's (a non-hostile chaser) has an unviewable face. This could imply that Sometsuki wasn't close to people in her school/had very few people to rely on.

Bullying Because of Envy
Many of the effects (Rabbit, Coat and Muffler, Cake, Shikkenken, Sea Hare, Android, Ribbons) makes most NPCs to follow Sometsuki. The possible reason is because a lot of people paid attention to Sometsuki, which prompted her classmates to get jealous and start picking on her out of insecurity

Red Water World
The Red Water World (the area where you get the Short Hair effect) is one of the darkest areas in the game. The scissor statue where you get the effect is blood-stained.

The chasers in this area represent people who bullied Sometsuki and at some stage forcibly cut her hair short.

Sleepwalking
One possible reason that Sometsuki won't leave her room is because she's a sleepwalker, as noted by her title in the menu. She may have had a history of waking up in weird places after sleepwalking, after which she started locking herself in her room to prevent that.

Sometsuki Was Betrayed
In the past Sometsuki had a girl-friend (aka the girl inside the green house in the Green Plant World who also gives you the Green menu theme) So, let's call her Midoriko (haha get it cuz Midori means green in japanese and ko is normally used in female names). Sometsuki and Midori were close, really close. It can be implied by the fact that Sometsuki can easily lie down on the bed while Midoriko sleeps on it. But one day, everything changed, when Sometsuki took a peak of Midoriko's Diary, it made Midoriko very...........angry lets just say that. Judging by the creepy appearance of Midoriko in the Uboa event (which literally looks like a giant version of this game's chasers), it seems that she was the one who instigated Sometsuki's bullying (as seen by the chasers of this game being corrupted high school girls and the short hair effect).