In the story "Scandal in Bohemia," who is the "slim youth"?
I assume you are talking about the slim youth who hurried by late in the story. The youth seems to be the one who called out a greeting (good night) to Holmes.
As it turns out, the youth was not actually a man. Instead, it was a woman -- it was Irene Adler. We find this out later in the story because she herself tells him that she was the one who had disguised herself and walked by him. She admits this to him in the letter that she left in her home for him after she left England.