When Denise was a kid, she thought Mary Poppins in the Disney movie was an angel. She didn't think much about Bert, she thought he was just a cool dude. Rewatching the movie now, both Denise and Rosy started wondering... what... exactly are they. They both seem to be magical in similar and different ways.
Thinking about it... we started realizing that both of them might be demons. Or maybe she's an angel and he's a demon -- explaining their differences.
They are both agents of chaos and change. During his tirade, Mr. Banks explicitly refers to Mary Poppins as a chaotic force disrupting his perfect house and his perfect schedule.
Bert does something completely different every day, but it's always exactly what he needed to do. He seems to be homeless and making a living by busking. One day he's a one-man band; the next he's drawing chalk murals that the children later explore; he's a chimney sweep the day Mrs. Banks wants to get the chimney cleaned; he sells kites on the wonderful windy day. Bert embraces chaos in all of its positive aspects.
Both Mary Poppins and Bert are magical. Maybe even immortal? It's never pointed at, but there's just this air that both of them have always been.
In the beginning, when Bert sings about the town people, it seems like maybe he is such a fixture that he knows everyone and therefore can sing about them. Or maybe he doesn't know them, and can tell what people's names are and what to sing about them through some magical means.
Bert commands a legion: the chimney-sweepers that appear out of all chimneys at command. Don't tell me that everyone in London was having their chimneys cleaned exactly at the same moment so they could pop up all at the same time... The chimneys could be portals from which the chimney-sweepers are summoned, sometimes in ways that defy physics - at one point, two chimney-sweepers pop out of a T-shaped chimney at the same time, even though, logically, they could not both fit in there.
Or maybe all the chimney-sweepers are different facets of Bert. Either explains how do they all seem to know Mary.
Bert and the other chimney-sweepers dance at dusk as the sun goes down. Seeing them as demons make this setting extra demonic. And they are all blackened with soot, like some descriptions of the devil.
When Mary Poppins initially won't let the kids go into the chalk drawing, Bert says, I'll take care of it myself. I'll do a bit of magic, he even says. He can't do it on the first try, but it's presented as, maybe sometimes he can. And it's implied later, when the kids go up in the chimney, that he did something magical. The kids imagine the rooftop world of chimney sweeps, and so they pop up there. Mary later chastises Bert for putting the idea in their heads. Bert must have the magic of suggestion.
At the end of the movie, he uses reverse psychology on Mr. Banks. Thinking of Bert as a demon makes it seem extra demonic when he is telling Mr. Banks things like, yes, be an asshole, of course you should. Bert of course intends the opposite, but maybe the devil always intends good, huh? Satan is not always portrayed as pure evil. In the Jewish perception in particular, Satan tempts and tests you specifically so that you can then choose to do the right thing.
As for what concerns Mary, although she starts out sitting on a cloud and seeming to descend from the heavens, she looks to be more aligned with demons and chaos and change. She's the friend of an obvious demon and his entire legion. She says cheeky things all the time. She trips you up and enjoys it. That seems more demonic than angelic. So maybe, rather than an angel, she might be a demon of the air. She rides the winds and seems to somehow control the way the wind blows through the changes in people's lives on Earth.
Can Mary Poppins see in the future? How did she know to put the idea of feeding the birds in the kids' head? Maybe she sees the threads of possible choices and knows how to nudge people in the right way? Is that the same thing that Bert did when he used reverse psychology on Mr. Banks?
By the way, is the bird lady also a supernatural being in league with Mary and Bert? After Mary Poppins tells the kids that they will meet the bird lady, they see her as their father is taking them to the bank. They ask Mr. Banks if he saw the bird lady, and he dismissively says that of course he saw her... but did he really? Can you only see the bird lady when you look for her and are pure in your heart?
Later, after Mr. Banks' epiphany, he looks for the bird lady, but she's not there at night. He has missed that particular chance to be kind, and this must leave an impression on him when he returns home to spend time with his family.
So, three supernatural beings of chaos all nudged this family in just the right ways to turn their lives upside-down... for the better.