I have known about personas for many years because I use them all the time. Long before I started writing I used different persona for different situations. Each identity is a mask that can be used as a replacement to emotions, especially if you are emotionally challenged.
The first major persona creation I noticed in life was day vs night. There are certain elements of my personality that is hindered by the day, and only the setting of the sun allow to emerge. These light and dark personalities were created early before my teenage years, but were confined to dreams. On my 13th birthday there was a battle in a dream and the one emerged victorious. After that it was two sides of a coin.
Years later I created personas for writing. I would write fanfiction stories and need to be everyone of my character to cover the angles. Of course this meant creating many kinds along the way.
It's actually a lot of fun, but you must have a strong mind to control them, or the personas will take over and you'll lose yourself. So be careful.