Dark Lillith

What makes a good portrait? I shot this outside at a mall, my brain could only see distractions. I used a 4 stop nd filter shot wide open and flash.Fairly new to this. When do you shoot wide open or stop down to F2.8 or F4? When do you include a sharp background? How do you include sharp background that is not a distraction? How did you learn to pose full body shots?
https://imadethis.photo/1/mall.jpg