Bert was 17 when he first fell in love with Mary Poppins, back when he was a young man, the prince of his house, and heir to his father's throne. He wasn't called "Bert" then, back in that time, back on that cursed world of cruel magics, but she was called Mary Poppins (and always is, no matter where or when she is). She was the nurse for his younger sisters, and he couldn't keep his eyes off her, couldn't stay away.
He became immortal the first time she kissed him. She did her best to warn him beforehand, to tell him what it would mean, to live forever, and how once his choice was made, there was no reversing it. But Bert was young and impetuous and in love, and nothing sounded better to him than to be able to live forever with Mary Poppins.
Now, several thousands of years and hundreds of worlds and too many lives to count later, Bert still thinks he made the right choice.
It was all very scandalous and hush hush, of course - Bert's parents threatened to disown and disinherit him for pursuing the relationship, When he refused to end it, when he declared his love for Mary Poppins to the King and Queen, their threats became reality, and Bert was no longer a prince, no longer their heir, in complete and utter disgrace for falling in love with a servant.
Bert doesn't think much about his family and the world he came from. His parents and sisters died so long ago, he's almost forgotten them entirely. Once he briefly visited that world, thousands of years after their deaths, and when he saw their likenesses in the Great Hall of the Palace, he didn't even recognize them at first - he hadn't remembered how cruel their faces were. Bert realized then that Mary Poppins had saved him from a terrible fate. The world itself died several hundreds of years later, but Bert never knew, not until Mary mentioned it had come to an end, and then all he felt was relief, to finally be free of that evil place.
But when Bert does think back, it still amazes him, that in that world of magic and wonders, only he saw Mary for what she was - someone different, someone special, someone more. And he thinks, really, in the larger scheme of the things, the scandal was not someone of his social standing and rank falling in love with a children's nurse, but that someone like Mary Poppins could fall in love with an ordinary person like him.
Bert's spent countless untold years with Mary, traveling with her from place to place, from family to family. Sometimes he arrives in the place before her, and waits. Sometimes he catches up to her, after she's been there for a while. He's immortal, of course, thanks to her, but he's not immortal like her. There are places Mary Poppins can live and thrive that Bert can't, and during the years when she's living with families under water, or deep in the heart of a flaming volcano, Bert can only wait for the winds to change, the tides to turn, the volcano to sleep, and hope the next child lives in a place where he can join them.
He's not magic like Mary Poppins, but over the years, Bert's developed his own magic, making paintings and drawings that can come to life, and can be used as portals to other worlds. When Bert's not traveling with Mary (and that hasn't happened regularly in years and years), he just sketches a quick image of where Mary's gone to, and hops into it, so he can meet her there. He's learned to talk to animals, to whisper with the winds, and sing with the stars, so he can get messages to Mary when they're separated.
Once, when she was living with a family under the sea, Bert found himself a job on a ship. He would watch from the deck for a passing whale or porpoise, and when he spotted one, he'd call to them, and ask them to deliver a letter to Mary. Of course, every creature in the sea knew Mary Poppins, and they were pleased to have the honor of delivering Bert's messages. He'd give them a letter or drawing that he'd folded up and tucked into a shell, and thank them for their help, then wait patiently for their return, with a reply from Mary.
Bert has drawn pictures of all the children that Mary Poppins cared for, in all the years they've been together. Long ago, in the beginning, when Bert realized just how old she was, and how long she'd been doing this, he asked her how she remembered the names of so many children, some long since dead and gone. Mary just gave him a sad smile, and said, "I loved them all. I could never forget them, not even one of them."
And after that, Bert started his sketchbook, each page a different child or a different family, because he understood that part of his role in this immortal life was to remember, to mark the children's lives and to mourn their passing, so that none were ever forgotten. He's not good with names, like Mary is, but he never forgets a face.
Bert, of course, has some favorites among the children - the funny Banks family in London, the multi-colored unicorn herd on the glimmering planet, the mischievous Hobbit lads and lasses in their maze of smials under the green hills of the Shire. And there were some he never quite warmed to, like the young shark prince (Bert couldn't help but be unnerved by his smile) and that odd family of seven siblings, each so focused on their own interests (although the youngest one was a delight). But when he flips through his sketchbook (so full of drawings now, but there's always room for one more - like Mary Poppins' carpetbag, Bert's sketchbook is much bigger than it appears), he smiles at every face, and all the memories each page holds.
Bert's learned (and forgotten) thousands of languages over the years, so he can speak to the people in all the worlds and lands he travels with Mary Poppins. He's not particularly good with accents, and sometimes he makes people laugh, but Bert doesn't mind that. He learned a long time ago that the best way to make friends with someone was to laugh with them. In fact, it was when Mary laughed at one of Bert's jokes that he knew she was falling in love with him, all those years ago.
Bert still thinks it's very lucky that Mary's never grown tired of him, never fallen in love with and kissed another, never created for him an eternal rival for her affections. He, of course, could never imagine growing tired of her. He's been in love with her forever, and while winds change, and tides turn, and stars burn out, Bert knows that is one thing that will never change.