How could I have forgotten to answer this question?
Pinoko was a twin who was actually forming within her sister's body. However, she was growing very slowly. When her sister was fully grown, Pinoko was large enough to be detected; however, she was no more than a mass of vital organs, lacking a body. BJ removed her (there was a bit more to it than this), put her into an artificial body, and tried to present her to her sister, who only knew that a tumor had been removed from her. She rejected Pinoko, & BJ kept her as his own.
Regarding her age: while she has both the appearance and the innocence of a little girl, she does know things from the time she was in her sister. Thus, she makes comments about adult love, but also is unaware that she has been abducted (see the 7 disc series).
