Calling it now, Sayaka will turn out to actually be related to Kusora; they might be estranged sisters or something, and have bad blood between each other, which is why Sayaka had the letter from her sister tucked away in such a strange place. They even look kinda similar, and have similar hair styles, though Sayaka's hair is short, and Kusora's hair is long. The whole "My relative worked for her and disappeared." story could be a lie, or at least a half-truth in order to trick the MC into luring Kusora out to kill her.