She is first introduced in season 4's flashback episode, "Fork in the Road". A few years before Mike Ross became a lawyer, Mike was working as a bike messenger. During a courier trip to a Social Services legal clinic in the city, he met Claire, an intern and receptionist who was struggling to understand visa's and immigration law. Mike offered to help her, and the pair later went out for dinner.
At dinner, she asked Mike what else he did beside bike messaging, telling him that if she were to be in a relationship with a man, he'd have to be more than just a bike messenger. Mike lies to her and tells her that he is a lawyer, and claims to study at Columbia, after she tells him she studies at New York University.
After successfully filing a legal form for Claire that would allow her client to remain in the country, his work is thrown in the trash by Claire's boss, who was attracted to Claire and threatened by Mike's involvement with her. He tells Mike to leave, as he did a background check on Mike; however, he also tells Claire. Although Mike tells her that he likes her and is changing his life around just for her, she shoots him down, saying she doesn't believe him and that she never wants to see him again.
After Mike Ross is officially made Junior Partner by Jessica Pearson, she hands him his first case as partner. Mike asks Rachel Zane to be the associate on the case, but backs off when he realizes the opposing attorney is none other than Claire. He leaves the case in Rachel's hands, but as Rachel is just a law student and a part-time associate (as she is a paralegal by profession), Rachel is unable to authorize any deals on the case. Claire, however, notices that the partner assigned to the case is a Michael Ross, and wonders if it's the same Mike Ross she once went out with years ago.
Rachel attempts to close the deal afterwards, but blows it once Claire points out that she is a newbie, which gets under her skin. As the deal involves Google, the firm is set to lose a lot of money, which not only makes the firm look bad, but Mike, as it is his first case as partner. Not only does Rachel have to prevent anyone from figuring out Mike himself is not working on his very first case as partner, but also to prevent Claire from finding out about Mike. Rachel later finds out that one of Claire's very own clients could benefit from the deal, and offers it as a win-win solution to Claire. However, upon returning to the office, Mike tells her that the company she made a deal with, GigaDyne, had just entered the defense contracting business the day before, and as a government supplier, will make a background check into the acquisition, as well as everyone involved.
As Mike's name is on the case files, Mike has Rachel erase his name from all the files. Claire, however, is aware that the files have been altered and asks Rachel why Mike's name is removed, and realizes that the name was removed due to the impeding background check. Putting two and two together, along with her initial suspicion, she realizes that the Mike Ross on the case is none other than the same Mike Ross she once knew, thereby knowing that Mike is a fraud. She plans to report him to the District Attorney, but relents after Rachel reveals to her that she and Mike are engaged and she begs Claire not to.
Mike, however, realizes that Claire knows the truth and approaches her outside her law firm. He thanks her for not blowing his secret, but she tells him she doesn't believe that he turned his life around or that he loves Rachel. Mike begins to question how she could ever say such a thing, but Claire tells him that if he really loved her, he wouldn't marry her, because one day his secret will come out and she would be implicated.