It's his younger sister, Kelly.

John could tell from the vague outline and the singing that it was his sister, Kelly. She was a little shorter than him (he was around 5'8"), and had long auburn hair that fell down past her shoulder blades. John himself admitted that she was very beautiful, and that if she wasn't his sister that he would have asked her out by now, no doubt.

But she was his sister...

John stood there in the doorway for a few, silent moments, watching Kelly as she showered.