Ruby Moon is Nakuru's true form. Together with Spinel Sun, she's Eriol's guardian, like Kerberos and Yue are to Sakura.
Ruby Moon she's got pink hair & red eyes. And is weird & funny, just like her disguised form.^^
The other Yue, but in service to Eriol. This is the magical form of Nakuru. You have to give some credit for those wings; they are truly amazing.
Are you starting to wonder if Eriol has a thing for butterflies?