In all probability the man standing off to the right is a detective.
Now, the reason he looks transparent has to do with the way they took photographs back then.
Of course today everything is digital. But not to long ago, say 15 years back or so. Cameras were mechanical. When you pushed the little button to take a photo a little door would open, let in light, the right amount of light, then close. Poof! your image is now on the film. The little door could be open 1/100 of a second 1/60 of a second etc. On 35mm cameras you could control the little door, called the shutter, depending what the amount of light that existed when you take the picture or how fast your film was.
When we took a picture with a camera you would hear a clicking noise. That was the shutter, or the little door opening and closing.
Of course, modern film was fast. That is to say, it recorded the image in a split second. Most good camera's had shutter settings of 1/2000 of a second for action photography. Very fast film
Back in Lizzie's day, film was slow. Very slow. This meant that when the photographer took the picture he would need perhaps 5, 8, 10, seconds or more, depending on how bad the light was.
So, let us say he needed 10 seconds. He would push the shutter. The little door would open, and it would take 10 seconds to record the image. If you were in the photo, it was very important not to move, or your image would not be clear.
If you watch movies of Victorian times, or a Western, you may see the photographer use a lens over as the shutter. He would put the film in the camera and when it was time to push the button, he would take the lens cover off count to 10, then place the lens cover back. Presto. image captured.
Now, if someone is in the photo for only 5 seconds, then walks out of the picture, the film would not completely catch him. So, if it was a 10 second shot, he could have walked into the picture at the 5th second or walked out of it after 5 seconds, and not be completely recorded, thus a see through image.