I love using back-alley stairs to create images for High School Seniors. The only drawback is that most of the time the light in these edgy areas is not very good. To help create beautiful light and control the darkness of the background, I use off camera flash (OCF). My lights of choice are a MoLight 360 and AD200.
The reason I love using MoLights is that I can shoot HSS (high-speed sync) and they put out a ton of light in a small, lightweight design. High-Speed Sync allows me to shoot wide open, in most cases f1.4 so I get soft, creamy bokeh that looks like natural light in areas where I can't use natural light.
I have the AD200 behind the senior to create an edge light on her hair set to manual at 1/16th power. Make sure that your edge light doesn't "spill" light across her face and only lights her hair.
The Fill light is a MoLight 360 set to TTL with -1 Flash exposure compensation. I always turn down the Flash exposure compensation so that my images don't have such a harsh flash look to them. The light is placed about 45 degrees to camera right and 45 degrees up from the subject. The car in the way didn't allow us to get the light in the perfect location as you can see by the catchlights her eyes but it is close enough. Many times I see photographers have the light too low. A good rule of thumb is to have the catchlights at 10 or 2 o'clock position in the eyes for good portrait light. Of course, rules can be broken but if you are just learning light, that is a safe way to start. I tend to 'feather" the light as well. I do this by raising the light and not pointing or angling it at my subject.