When Blair and Serena begin at Columbia in The Undergraduates, they seek out the key master only to discover it is Juliet Sharp, Nate's new girlfriend. She has a key for Blair, but tells Serena that it's her last one.
The next day, Serena sees another girl get one and realizes Juliet tried to keep her out on purpose. At Fashion's Night Out, she stages a fight with Blair and they expose Juliet for what she did. As a result, Juliet loses her position at the house.
In Goodbye, Columbia, Nate is studying at the house when Juliet comes to surprise him, and they have a party there that night.