Sidney Marks is a resident of Ugly Naked Guy's building. The character is not heard seen on screen, but interacts with the main characters by phone.
In "The One With The Evil Orthodontist", the gang notice someone spying on them with a telescope. When Joey gets Sidney's phone number, he is surprised to learn that she is a woman. Sidney turns out to be extremely charming, compliments them, and makes them forget about her spying on them.