User blog comment:StarMiya/Mysterous girl/@comment-3487190-20180126023547

In this series, sounds like only after you begin your own duo unit could you become top idols. So in this case, the untold girl seems to likely be a rookie instead of already a top idol, unless her partner is missing for certain reason.

My guess is that she's a barrel of laughs and a talented girl who works hard for quite a time, but in the beginning of story she fails to find anyone to pair with, often ends up lonely.