I'm pretty sure Yoshida is YoMi. There is evidence to back this claim, albeit not as much as I would like. My first peice of evidence is, of course, the names. Yoshida's surname is Hoshimiya. The Yo in YoMi obviously stems from Yoshida in this, while the Mi I believe comes from HoshiMiya. Obviously that is not nearly enough evidence on its own, but it is something as she is the only character we know of so far to have YoMi in their name. My second piece of evidence is that YoMi is a student of whoever the letter was addressed to. Since the letter is found in Shelly's mansion, I believe it is safe to assume the letter was addressed to Shelly, A.K.A: Phyre. This is also backed up by the fact that Wendy can easily decipher the code. Since Wendy proclaims to be a part of Phyre, it would make sense why she would be able to decode something that was sent to her.Who just so happens to be a known student of Phyre? None other than Yoshida. My final, and possibly most circumstantial, piece of evidence is that YoMi claims to have not wanted whatever was given to her by her teacher, but was accepting of it in the long run. Phyre was known to expirement on people as seen with Marianne and Isabelle. It is highly likely that Yoshida would have also been expiramented on by Phyre, and it would make sense why she would not have wanted to be expiramented on, even if the results were in her favor. While there is not enough proof here to say definitively YoMi is Yoshida, I'd say it is definitely a good possibility. That said, why a letter from Yoshida would be in Erica's memories is hard to say, but it probably has to do with why Wendy and Shelly are in Erica's head as well.