Milano was born as the daughter of Sylvia and Robert Milano, but her father died when she was very young trying to kill the mortal demon Azazel, who Milano herself later learned about from the clues left nehind by her father, as well as learning of his fate from her mother. Her religion is appearingly connected to the Catholic church, and she states in the film that she believed a few demons, including Azazel, were deprived of form and made mortal when God cast them down after Lucifer's rebellion, and put a few humans on Earth to fight them. Milano grew up to become a theology teacher at a college, living alone in preparation for a possible confrontation with Azazel. Her mother is now dead.
Gretta Milano made her debut in the film at her apartment, where she was met by Detective John Hobbes, who wanted to ask her about her father, Robert Milano, who had committed suicide thirty years prior. She agreed to talk with him under the grounds that what she said be just between them, and explain how her father had once been the pride of his police force, before catching a killer(one possessed by the demon Azazel as it happened), and being framed for murders he didn't commit after a copycat killer then appeared. Milano couldn't explain how she knew her fatehr was innocent, but stated she would like to see the real killer punished before Hobbes left. She is later met by him again after he'd gone to where her father committed suicide, her mother's family mountain home, and informed her so, before asking Milano what "Azazel" mean't. After hearing this, she advised him to simply not take this case if he had family and people he cared about, but Hobbes answered that he had no choice as it was his job. She is not seen again until Hobbes comes to her after a fated meeting with Azazel, wherein she finally revealed that her father had also had an encounter with the demon and had been framed for murder by him. As they talked, Milano explained the nature of Azazel and all demons like him, as well as her religion, before leaving after answering that the reason why she lived alone was because of all this, and how she didn't want to put anyone that might get close to her at risk. Afterwards, Azazel, in his current host body, followed her and, when he cornered her and seemed to remeber her from the past, she made a run for it, knowing it was him, and got away in a taxi. At a Catholic church later, she wa smat by Hobbes, who had come at her request, and told him of her encounter with Azazel, and how she hoped secretly it would never come, but seemed sure he didn't remember her completely. At this point, Hobbes declared they shouldn't be seen together anymore. She later called hima t the station after Azazel had possessed a teacher and forced Hobbes to kill him, before he said they'd talk after he got home.
When he called her back, Milano explained to him what Azazel and all the demons like him wanted; the Fall of Babylon(civilization). Somewhat surprised when he revealed Azazel simply moved into another host after the teacher died, she exclaimed after hearing him say that he'd read from one of her father's books that the demon can only live for one breath only outside of a host that there was an ancient Hebrew text that said the breath could carry them for 500 cubics, Nobbes asserting from this that she was right about them being limited in certain ways, before they both hung up. The next evening, Hobbes and his nephew, Sam, arrived at her apartment, having no place left to go, and she let them in, greeting Sam and getting Hobbes a cup of coffee. Later, she listened to his figure on Azazel, with her exclaiming the reason why he can take over anyone in spirit form, even someone he couldn't by touch, was because he would then be fighting for his life, and would therefore be that much more powerful. Asking him what he planned to do, Hobebs answered that no matter what he did, she was to take Sam somewhere nobody, even himself, knew about in order to be sure they would be safe. When he prepared to leave, Milano hugged him goodbye, telling him she was ready. Though she was niot seen again for the rest of the film, Milano presumably followed Hobbes' command and never learned of his death or failure to kill Azazel.
Milano is a very polite and nice woman, but at first appeared secretive and a loner, this due to her life of preparation against the demon Azazel. She stated in the film she'd never want to put anyone else at risk by sharing this part of her life due to the demon's homicidal nature. However, when she met Azazel in his host, she became fearful and fled from him, unable to do anything to stop him. After she met Detective John Hobbes, howeer, though she initailly tried to avoid having anything to do with him, she developed a somewhat strong bond with him after Azazel was revealed to have taken an interest in him as he had done her father, and she helped him along with his plan to get rid of the mortal demon. She also willingly agreed to care for his nephew Sam afte rhe left to execute his plan to kill Azazel, despite the fact that he may never come back.