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.