The distinction between Billy, Freddy and Mary Batson and Captain Marvel, Junior and Mary Marvel
Just some random examples of the way the Marvels were separate beings from the kids who summoned them. I particularly like the argument between Cap and Billy; Captain Marvel has fallen in love, stays out all night and neglects his duties, and Billy is ticked off enough that they start arguing by switching back and forth. The kids retained the Marvels' memories and vice versa, but they didn't think the same and reached different decisions on matters.