What was Bert?
He knew Mary and went with her on her adventures but didn't seem to perform any magic of his own or if he did it was extremely subtle.
If this was a J.K. Rowling story he'd either be a muggle who knew of the wizarding world and not only accepted it but embraced it and was therefore in Mary's good graces because he gleefully went along with every whimsy she conjured up.
Or he was a squib who could do limited amounts of magic and aided Mary with it.