Hmm. Well, "watashi" is used for any formal speech by male or female alike. Although in this setting, and with them being teens and all, I guess it can be quite controversial since every other male was using "ore" or "boku".
Either way, I wonder what Sui meant with "alike". Alike in a way that both of them likes girls...or Miyu? I'd like them to be allies tho. Maybe a plot device to make Miyu jealous or smtn.