Extract person from video
If you have a frame where the person does not exist, you can use a tool called a difference matte. (You'll need to scroll down to the section titled "Difference Matte Effect" for a description.) It will find a difference between the frame without the person, and the frames with a person and remove areas that are the same, leaving only the difference (in this case the person). As linked, After Effects has such a tool. Many other video editors have it as well.