Millie was born to John and Mary Winchester named after John's mother. At the age of 2 and her older brother Dean being the age of 4 was kidnapped one night by Mazikeen, a demon, ordered by Lucifer/ the devil who somehow communicated with her from his prison. Maze takes care of Millie till she's 6 until she's taken by Amenadiel,an angel, who tries to take her to heaven until the plan was compromised by archangel Gabriel who takes her and leaves her at an Orphanage in Liverpool, England where later when she's 20 she meets a 25 year old John Constantine of which is now her best friend and to their surprise 'soulmates'.. supposedly When she's 26 she goes off the grid hidden from everyone who cares for her who all think she's dead now for being gone for 2 year now.. Except for John because he still believes she's out there except he feels deep down inside of him she is not only hiding for her loved ones but.. Something bigger and much stronger.... Her destiny.