Kip has been given two histories on Friends. In both he was Chandler's roommate before Joey. In "The One With The Dozen Lasagnas", Chandler tells Joey that Kip ran off and got married. Later, in "The One With The Kips", Rachel says that Kip and Monica dated and after they broke up, he was phased out. In "The One With The Blackout", Joey says that "Chandler's old roommate was Jewish".
When putting it together, it is possible that he was Chandler's Jewish roommate who dated Monica before they broke up. After which, despite them all promising to remain friends, he was phased out and then he ran off and got married.