If/theories

Shimitsuki's Name
In ED2 and ED3, we know that her real name is Izumi, Shimitsuki would probably mean death (死, Shi) and body (身, Mi) which speculated she had died in a train accident, and also mean to be 4 (四, Shi) and 3 (三, Mi) because 16:03 (4:03 pm) was the time she died.

Shimitsuki's Effects
Shimitsuki's effects description will change into a word of Kanji in other meanings when she is in her room. Speculations part is a WIP!

Shimitsuki is Virtual Human Being
Shimitsuki in fact is not real, she been made by girl with pigtails from Monochrome Villa, Osoro, so judging by End 3 she made Shimitsuki like Artificial Intelligence, but something goes wrong with her and Osoro decides delete her creation. Proofs is, when Shimitsuki using Bass effect she can turn in girl with pigtails and violoncello, and who is pigtails girl? Correct its girl from Monochrome Villa aka Osoro this part can be backed up by description in real for Shimitsuki world, puppet, its like Shimitsuki is puppet and Osoro her puppeteer, haircut of Shimitsuki in Matching effect same as Osoro's haircut, and it strange that Shimitsuki have a Noise effect, its like Shimitsuki have some bugs and errors, like in computer program, probably from this bugs Osoro decides delete Shimitsuki/Izumi.

Colorful Girls
Colorful Girls, Non Playable Characters which looks like neon girls in multicolored dresses, mostly they playing role of transporters to different locations, total number of girls is six, but question is who those girls are? Well, judging by third and true end, where on the path to Osoro can be seen human portraits of colorful girls, those girls can be artificial humans just like Shimitsuki, but they been made from friends or classmates of Osoro.