This chapter where Miki displays her reactions are very funny! I can really imagine how she freaks out and thinks of all these crazy poses/things! I really can't guess who this mysterious girl is...why won't Yuko be happy to hear about that? There is no clue....or is there???
HINTS please!!
Okay, I'll help you out a bit.
Miichan actually has a secret she's keeping from Yuko, which also has to do with Mariko(or Miichan's "girlfriend" as the person she was talking to on the phone mentioned.) We don't know who it is yet, but now we know Miichan has close connections with a very important person.
As for the girl, she is important because she is connected with Mako, so the vision of her appeared in the dream mirror.
If you wanna know more, then please wait for the next chapter.