Emily goes all out to greet each and every individual visitor to the party even if she doesn’t like them. That makes it harder for non-classmate Kylie to get in. In fact, if Rain was dressed as anything OTHER than an angel, I don’t think he would’ve gotten in at all…
Fun fact: considering Frankenstein was the doctor's surname and the monster is described as his son, it would make sense that the monster shares his surname, thus making both the man and the monster appropriately "Frankenstein".
I can't wait to see if Maria corrects Emily on Rain's costume.
My guess is they've got their costumes backwards with him and Emily