Ok so first of all a lot of these are still not answerable, and will probably never be answerable since I doubt Game Freak would realize Back Story about a character who is generations old.
Who is she: a random ghost girl who, according to the Generations episode, attacks innocent travelers.
Parents: Not known and probs never will be.
Why is she appearing in every gen: Kind of like an easter egg, or maybe Game Freak was just like, "How about we make 4 year olds feel like they are being haunted?"
Where She came from: We don't know, and probably never will, but I assume she died in the Old Chateu with her butler after they got lost in Eterna Forest.
Her Origin: According to swumwun, an Origin is a place where a person comes from, so I answered that.
Old Man: Her butler
Sources: Other answer