Isaac is a minor character appearing in two episodes; The One Where Ross and Rachel Take A Break and The One With The Morning After. He is Jasmine's brother and Chloe's co-worker.
Isaac is first seen when Joey and Chandler go to the Xerox store with the intention of hitting on Chloe while they are there. Thier strange, wacky behavior and comments give away thier intentions to Isaac, who tells Chloe they are there to hit on her. Later, Chloe tells him off-screen that she had sex withRoss, who at the time was drunk and mistakenly believed that his relationship with Rachel was already over. Isaac seemed to know who Rachel was, as did his sister Jasmine (presumably, Jasmine met Rachel while working at Central Perk, as Jasmine was working there as a waitress throughoutThe Pilot). After Ross learns that his relationship with Rachel was in fact intact when he had sex with Chloe and tells Joey and Chandler what he has done, they convince him not to ever tell Rachel, not wanting either Ross or Rachel to get hurt, but at the same time Chandler works out that if Chloe tells Isaac about the fling, he could tell Jasmine, who in turn could tellPhoebe, (Jasmine at some point apparently quit her job at Central Perk and started working alongside Phoebe at the massage parlour) who in turn could tell Rachel. Ross runs down to the Xerox store, only to find that Chloe has already informed Isaac of the affair. He speaks to Isaac, who greets him by exclaiming, in an amused tone of voice, "You dog!" He seems to agree that it's best if Rachel doesn't find about the sexual encounter Ross and Chloe had, stating that he and Ross need to look out for each other because they are "the same." He seems to be implying that he too once cheated on a woman he was in a relationship with, an implication which irritates Ross, who angrily argues with Isaac about whether or not they really are the same, and ends up screaming at him that they are not. He then asks Isaac not to inform Jasmine of the affair he had with Chloe, only for Isaac to respond by saying "I can promise not to tell her again." Although Ross is able to get to Jasmine before she can speak to either Rachel or Phoebe and convince her not to let either of them know about his fling with Chloe, he is unable to stop her from telling her roommate about it-and her roommate turns out to be Gunther.