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 and has been made by girl with pigtails from Monochrome Villa: Shizuku. So judging by End 3, she made Shimitsuki like an Artificial Intelligence, but something went wrong and Shizuku decided to delete her creation. Proof is that when Shimitsuki is using Bass she sometimes can turn into a girl with pigtails and violoncello. And and who is that pigtails girl? Correct, it's the girl from Monochrome Villa aka Shizuku! This part can be backed up by description for the Effect in real world, puppet, like Shimitsuki is puppet and Shizuku is her puppeteer, and the haircut of Shimitsuki in Matching Effect is same as Shizuku's. Also it's strange that Shimitsuki has Noise Effect, like some bugs and errors in computer program, probaby from where Shizuku decides to delete Shimitsuki/Izumi.

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