Bert (at least in the movie) is as magical as Mary.As a jack-of-all-trades, he's clearly a master of at least a few; and he has a unique sense of perspective. He and Mary have "history", and he always seems to be in just the right place to help out or liven things up. He's always in just the right job (even if the kids don't know it yet) at just the right time. Good-natured happy-go-lucky drifter or Chessmaster of the entire MP cosmos? You be the judge.
One of Bert's many jobs is posing as Mr. Dawes, Sr.Bert adopts many jobs out of narrative convenience; but throughout the film, his main duty is to demonstrate the folksy wisdom of the underclass. Perhaps he also puts on a bald wig and a fake beard in order to demonstrate the corruption of the upper class. Plus, have you ever seen Dawes Sr. and Bert in the same room together? Think about it.
Bert is a member of the Dawes family.He might even be Mr Dawes, Jr.'s son, making him the grandson of Mr. Dawes, Sr. There is a family resemblance, though obscured by Dawes, Sr.'s advanced old agenote . Always more of a free spirit than the rest of his family, making him quite the black sheep being from a long line of bankers, he ran out and was summarily cut off from his family fortune. This was quite all right by him, as he obviously prefers getting by doing exactly what he wants. He takes great pains to distance himself from the rest of his clan, going so far as to affect a terrible Cockney accent so as to sound nothing like them.
The woman who feeds the pigeons is Mary's mentally ill mother.Very few people know about her, and Mary looks awfully wistful as she sings about her. Why would Mary Poppins, who has spent nearly all of her time as a magical nanny, know about this obscure person? She knows of her because they are related.
The Bird Woman is an older version (a much, much older version) of Mary.It's implied that she can communicate with the pigeons, or at least believes she can. Mary can communicate with them as well. Mary-from-the-future came back to make present-day Mary's lesson to Michael more effective.
The Bird Woman is a Saint.She's the Earthly manifestation of a charity and compassion. When the children refused to feed the birds, she struck back by causing a run on the bank. She's not malicious, just somewhat playful with an odd sense of humor. Mary's line about the "Saints and Apostles" watching was literal. If you show that you care, they smile upon you. If not, then you're in trouble.
The Bird Woman died.She's a homeless person on the streets of London; it's highly unlikely that she would have survived for much longer. I know Mary says that the Bird Woman appears "early each day", but there's just something about Mr. Banks's body language as he looks at the empty steps of the cathedral.
Mary Poppins is a God trapped in a human body.No amount of witchcraft could produce an entire universe made from a chalk drawing. Clearly, Mary is a deity.
Mary Poppins did not exist before the events of the movie, and was created Hogfather-style, by the children.Probably every child who has ever had a nanny wishes for a "practically perfect" nanny. In the Discworld book Hogfather, it is shown that if there is enough free occult space, anthropomorphic personifications can be created by someone stating that it exists and others agreeing that it makes sense. Jane and Michael's song perfectly described Mary, and, combined with thousands of children's worth of belief, created her.
Mary is immortal.Because she's a witch.
(connected to the above) Mary was Bert's nanny.
Bert is a wizard
Mary Poppins has a different guy in every townHer job is very transient, and so it may be that Bert is not the only man in her life.
The entirety of Mary Poppin's visit is dreamed up by the childrenMary Poppins is simply a normal (but better than usual) nanny who somehow managed to catch the children's interest. They dreamed up some fantastic adventures and thought up all these things.
Mary Poppins is the mother of Madame Pomfrey.They're both magical British caregivers.
Mary Poppins and Nanny McPhee are sistersThey're both magical and fairly no-nonsense. They both started their nanny careers together, mysteriously showing up out of nowhere to help families with rambunctious children and parents who have troubles communicating with them, and then leaving as soon as everything is fixed. However, due to differences in opinions about what sorts of disciplinary actions should be taken, they went their separate ways.
Mary Poppins is the mother of Maria Von Trapp.Think about it, Mary was a young woman during the Edwardian Era, which means she would probably have been in her thirties during the end of WWI, when Maria must have been born if we assume she was in her twenties when she married Captain Von Trapp in 1938. They both share a similar apperance, a similar singing voice, and a similar gift for being the nanny for children that have previously scared away several predecessors.
In the books, Mary Poppins is NyarlathotepThe movie depicts her as kind and loving, but in the books...
Mary Poppins is on the same wavelength as Willy Wonka, and possibly his mother.Mysterious benefactors of lonely children, both seem to exist to teach both parents and children the benefits of etiquette, temperance, and honest self-expression. Furthermore, both seem more than a little hung up on the spiritual applications of sugar, and both seem to have the power to transform foodstuffs into various colors and flavors at will. Mary Poppins' love of travel and wide circle of acquaintances made Wonka come to distrust the outside world, withdrawing to his magical factory. I would not be surprised if Bert's last name is "Bucket".
Mary Poppins worked for HogwartsShe went around Wizarding England finding muggleborn magicks and accustoming them and their parents to the wonderful weirdness of the wizarding world. She trains them not to speak of their adventures to anybody else, hence her denial post chalk drawings. The winds changing is just her getting an owl off screen telling her there are some more children who need her service. She was probably in Ravenclaw when she went to Hogwarts and excelled so well in Muggle Studies and muggle interactions she was offered a full time job. Bert is a squib, in touch with the magical world and an old friend of Mary Poppins, but has no talent himself. All his tricks come from Magical objects, such as the chalk and the sweepers. Uncle Albert's laughing disease is a real wizarding sickness, Mary wasn't kidding when she said it was contagious. It's caught the same way as the common cold and the symptoms are activated by laughter. Michael passed it on to Mr. Dawes when they shook hands, but it took a while for the symptoms to manifest themselves as he had no sense of humor. Sadly, Mary Poppins is never mentioned nor seen in Harry Potter canon because she was killed during the first rise of Voldemort for her unacceptable fondness for muggles and muggleborns. The children whose lives she touched passed on her story until it got to Walt Disney and her abilities greatly exaggerated over time and for appeal for the film, so the children actually go into the pictures instead of watch them move around and Mary sits on clouds. You can go cry now.
Mary Poppins is related to ElphabaThrough the movie, a lot of screen time is devoted to Mary Poppin's feet. Fetish Feul or not, her feet are always pointed in opposite directions...just like the Wicked Witch of the East.
Mary Poppins is an alcoholicRum punch anyone? Mary never takes the children on any adventures unless you count the ones down to the local gin shop or liquour palace. The trips into the street drawing and tea on the ceiling with Uncle Albert are booze induced fantasies. The time she spends surrounded by dancing chimney-sweeps is symbolic for what is really going on (something that can't be shown in a children's film).
Bert has a speech impedimentHis condition is similar to foreign accent syndrome, but is milder, and was caused by moderate parental abuse rather than head trauma. He tells us all about this in "Supercalifragilisticexpialidocious":
Because I was afraid to speak when I was just a lad
My father gave me nose a tweak and told me I was badThis is why his Cockney accent is (seemingly) so awful.
Bert and Mary Poppins are siblingsThey are clearly extremely fond of one another, but not in a particularly romantic way; instead, they are siblings.
Mary, Bert, Nanny McPhee, and others are a part of a race whose purpose is to help children (and others while they're at it) learn something.There aren't many rules. The members can work how they want to and leave how they want to. The only rule is that they must use their magic to help the children. Bert still hasn't gained much skill at magic, which is why we haven't seen him use it much. So Mary's boss (whoever it is) told her to become more or less Bert's teacher.
Mary, Bert, and possibly other characters are Anthropomorphic Personifications.Mary would be a personification of Proper Raising of Children; Bert of Taking Joy in Your Work; the Bird Woman of Kindness to Animals. (Mary reveres her because Kindness to Animals is a much bigger thing to be an Anthopomorphic Personification of). Uncle Albert is another one, though I'm not sure what.
Mary Poppins is a witch.Specifically, her domain is laughter. Mary Poppins isn't her real name, but one she received after gaining her powers.
Mary Poppins is a Puella Magi.The chalk drawing was a Witch barrier, possibly that of Izabel (the Artist Witch) in another timeline, and Bert was kissed by the Witch. She needs Tuesdays off to gather Grief Seeds and slay Witches.
Mary Poppins was Bert's Nanny
Mary Poppins came from a dysfunctional familyLook at it this way. She, like everyone else (according to the books) was born with magical powers, but her family was laughably crazy (remember Uncle Albert?), so when she got her baby teeth, she was allowed by God to retain her powers that will help her survive childhood. She had a terrible experience with her magical powers throughout her childhood that caused her to become stern and almost uncaring. So why did she become a nanny? She didn't want other children to experience what she went through as a child. So she only went to troubled families similar to hers, where she manages to change them before leaving. And based on her appearance and personality, the Banks family may not have been the first family she "saved". Though it still does not explain why she's a Mary Sue, though.