Anna Milton originally appeared as an amnesiac angel who had chosen to lose her grace and become a human. Due to her newly-reawakened ability to hear angels speaking in Season 4, she was being pursued by demons and angels alike. With the help of the Winchesters, she regained her grace and became a rogue angel, helping where she could in stopping the Apocalypse, but she was eventually captured by angels with the help of Castiel, and imprisoned in Heaven. After the release of Lucifer, Anna attempted to kill Sam Winchester to prevent Lucifer from using him as a vessel, thus effectively stopping the Apocalypse. She was killed by the archangel Michael.
Appearance
Anna's human body was that of a young woman with pale skin, long red hair, and hazel eyes who was 5'6". She wore a pair of white and blue scrubs at Connor Beverly Behavioral Medicine Center; once she escaped the mental institution, she began wearing a frilly button-down white shirt and jeans with a green jacket, the last of which she later swapped out in favor of a navy coat that went down to mid-thigh.
Personality
Anna was dissatisfied with her existence as an angel. She disliked the strict rules governing her, and also admitted to not understanding God. While at first she longed to return to Heaven during her station on Earth, she eventually risked the wrath of her family by falling for the sake of happiness. She resented her siblings for their unwillingness to be more open to feeling and free will, but also felt lonely and wished for them to join her.
Unlike many other angels, Anna viewed humans as superior to her own kind; she especially valued their capacity for emotions. Her attachment to humanity was one of her strongest traits: she tore out her own grace to experience life as a human, she confessed to Dean that she would have preferred not to turn back into an angel, and she betrayed Sam and Dean in the hopes of stopping the Apocalypse which would result to saving billions of lives.
Anna was earnest, well-mannered, and trusting: she believed Ruby, a demon, to be good after she saved Anna's life, and repeatedly went to Castiel despite the fact that he had orders to kill her because she didn't believe he would even try. She offered a guilt-ridden Dean compassion and forgiveness for his actions in Hell and for seemingly betraying her in "Heaven and Hell". She was a religious person who apparently attended her human father's church. Although she initially feared retribution from Heaven (particularly God) for her disobedience to such a degree that she had to go to a psychiatrist as a child, she eventually became happy and well-adjusted with many friends and a promising future once she repressed her true identity.
After Anna started to hear angels, she became timid. She was relieved to learn she wasn't insane. The circumstances surrounding her distressed her, especially as she didn't understand what was going on or why it was happening to her.
Anna became more authoritative and reserved upon regaining her memories and moreso when she regained her grace. Once she made a choice, she followed through with it, which she described to Castiel as "confusing" and "terrifying." She preferred having the element of suprise, which helped her defeat Uriel. She was willing to commit extreme actions for the greater good, such as killing her own brother to save another, at the same time preventing that brother from serving Lucifer, and slaughtering the Winchester family to undo the Apocalypse.