Isaac is first seen when Joey and Chandler go to the copy store with the intention of hitting on Chloe while they are there. Their strange, wacky behavior and comments give away their intentions to Isaac, who tells Chloe they are there to hit on her.
Later, Chloe tells Isaac off-screen that she had sex with Ross, who at the time believed that his relationship with Rachel was 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 throughout The Pilot). After Ross learns that Rachel wanted to be his girlfriend again 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 tell Phoebe, (Jasmine is now working alongside Phoebe at the massage parlor) who in turn could tell Rachel.
Ross runs down to the copy 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 cheats on women he is in a relationship with, an implication which irritates Ross, who angrily argues with Isaac about whether or not they really are the same. 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, her roommate, Gunther from Central Perk, already knows about it, and tells Rachel.
Isaac: It doesn't matter how much we love them. Monogamy is too cruel a rule.