Come to think of it, in the various Idolmasters, we've got several idols who are questionably/deniably supernatural. It starts with Takane, who's maybe an alien, and then Cinderella Girls adds Noa, who is maybe a time-travelling robot; Eve, maybe a Santa; Chitose, maybe a vampire; Syukou, maybe a kitsune; Koume, maybe a medium; Yuko, maybe an esper; and Yoshino, maybe a god. (I'm leaving out obvious poseurs like Ranko, Nana, and Miku.)
Now, I don't know much about SideM, Million Live, or Shiny Colors, so I'm not sure — is there an idol who's been played as possibly a ghost? (I guess the "Karen is dead" bit could get played that way, but I've not seen it done.) Because if not, "that girl" needs to make her debut.
Mirei-chan...?It was just... that girl~...I thought something weird was going on, but it was just that girl after all~So that's what it was~!Secretly, that girl was haunting you... she said she had fun too...♪So I heard...Even Producer got in on the action and went all out during the pillow fight...We were going til late in the night!--Anyway, that trip was so fun!On a later dayWhat the heck~! Take this-What?Hey, Syoko! Grabbing my leg is a foul, got it!Whoa-!?*grip*Get ready for a max power pillow bullet...Huh?Don't aim for my blind spot, Nono!Oof-!?Producer! Let's go in for a super fast concentrated attack-!BAM, BAM BAM~!!