Who is Bert to Mary Poppins

Some have preposed he was a previous child in need of a babysitter. I disagree. There is a cheeky and flirty familiarity to them. "Supercala..." Sounds like a nonsense game they played when young. Bert is none plussed by her magic, nor the floating laughing fits of his Uncle. He himself is able to read the wind change, and do rather odd stunts with chimneys. I prepose that he may also be a person capable of some subtler magic, possibly half witch half human which is why his magic isn't as potent as Mary Poppins, and that they were childhood friends. It would explain why both know each others temperaments and the area rather well.