Kip is an often referenced, but never seen character on Friends. He was Chandler's old roommate before Joey.
There is one episode title that references Kip, "The One With The Kips", where the plot revolves around Ross telling Rachel he cannot see her anymore because of Emily's wishes.
History
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.