Jim Nelson was seen in the episode "The One With The Tea Leaves" as Phoebe's date. They met by seeing each other everywhere while out on their errands and eventually he decided to ask Phoebe out on a date, despite being slightly bemused when she misunderstood the pretty obvious meaning of a remark he made. On the date, Jim fell away from his initially polite and friendly demeanor and, seemingly just to amuse himself, started making highly inappropriate and rude vile comments which Phoebe found both disturbing and intimidating. He also claimed, most likely lying in yet another attempt to intimidate her, that he wrote "erotic novels for children" which were "wildly unpopular". Phoebe later referred to him as the "rude guy".
Jim appears to be a nasty, unpleasant, rather manipulative and perhaps misogynistic individual who could possibly be considered an antagonist, as he in a way bullied Phoebe on the date, so much so that for the rest of the episode she avoided going to the coffee shop and all the other places she had seen him while out on her errands in order to avoid encountering him again. Oddly enough however, she seemed comfortable going to the coffee shop without having to worry about him in later episodes. The reason for this is unknown, but it's possible that her friends may have confronted Jim and intimidated the disturbing bully into leaving Phoebe alone (as Joey, Chandler and Ross had vowed to give a man who had cheated on her the "butt kicking of a lifetime" in an earlier episode).
Apparently, he was based on a real life boyfriend of one of the creators.