keep a good hold

In his book, At the Back of the North Wind, George MacDonald depicts the following interaction between a little boy named Diamond and the mysterious creature called the North Wind.
After convincing the boy to follow her wherever she goes, the North Wind speaks-

"If you see me with my face all black, don't be frightened. If you see me flapping wings like a bat's, as big as the whole sky, don't be frightened. If you hear me raging ten times worse than Mrs. Bill, the black-smith's wife- even if you see me looking in at people's windows like Mrs. Eve Dropper, the gardener's wife- you must believe that I am doing my work. Nay, Diamond, if I change into a serpent or a tiger, you must not let go your hold of me, for my hand will never change in yours if you keep a good hold. If you keep a hold, you will know who I am all the time, even when you look at me and can't see me the least like the North Wind. I may look something very awful."