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.

Shimitsuki is Virtual Human Being
Shimitsuki in fact is not real, she been made by girl with pigtails from Monochrome Villa, let's call her Viola, so judging by End 3 Viola made Shimitsuki like Artificial Intelligence, but something goes wrong with her and Viola 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 Viola this part can be backed up by description in real for Shimitsuki world, puppet, its like Shimitsuki is puppet and Viola her puppeteer, haircut of Shimitsuki in Matching effect same as Viola'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 Viola decides delete Shimitsuki/Izumi.