Eponymous heroine of a series of books by P.L. Travers, originally conceived as an illustration of
progressive education in action. While her primary time period in the books is
Edwardian England, it's been revealed in interviews that Poppins is actually a female warrior from pre-Roman
Britain, made immortal through a pact with an unspecific
goddess. To Poppins's pleas to be spared the fury of the Romans, the
goddess's response was to make her the guardian of all earthly children, though she would never be able to mate or bear children of her own. Henceforth, she spent the past two millennia or so drifting from family to family, being the perfect mom to other peoples' children.
The Disney version played up her counter-cultural aspect, (after all, it WAS the Sixties, and this was, after all about the country that hatched The Beatles) without dealing with her more moralistic aspects. However, it was widely trumpeted at the time that Mary Poppins-the-movie was going to be Disney's first foray into a "real" Broadway-quality musical (Julie Andrews was quite the hot property at the time and musicals were much, much more of an art form than they are now, "Dancer in the Dark" notwithstanding). Somehow, that makes up for a ham-handed portrayal of women's suffrage and the fact that the film makes bankers seem like evil people who steal money from savings accounts... thus traumatizing quite a few kids into spending their birthday money, instead of banking it (like good children do, of course). And, of course, that the chimney sweeps would have either been not-so-privileged children or castrati.