Yep! Ruby Moon and Nakuru are the same person. (Some of you might already have known this....) In the same way that Yue has another form in Yukito, so Ruby Moon also has a different form. Although, in Yue's case, it seems to be a little more complicated than it is with Ruby Moon. Yue was dormant, sort of, for a long while...and he was kind of hiding within Yukito. It seems that Nakuru is just a less powerful form of Ruby Moon, who looks a little different and goes to school during the day just like Eriol does. Nakuru and Ruby Moon have the same memories and personality.
Nakuru and Ruby Moon also have
a little secret that you can't tell just by looking at them. Even
though both of Nakuru's forms look and dress and act like a girl....in
reality, she's a he. Don't you just love Clamp?