Colin Forrester is a charismatic billionaire guest professor at Columbia University who briefly dated Serena van der Woodsen.
Colin first appears in Goodbye, Columbia, as he runs into Serena each morning while depositing different women into taxi cabs. The two eventually become friendlier to each other and spend a night together talking. In Easy J, Serena begins her Psychology of Business class and discovers Colin is her professor. Because of their mutual attraction to each other, Serena agrees to drop the class until Lily uses reverse psychology on her to get her to change her mind. At the end of the episode, it's revealed that Colin and Juliet are cousins and he gives her money to financially support her.
They decide tor remain friends for the rest of the semester until they can date for real, but find this is harder than anticipated. In War at the Roses, they attend Blair's birthday party. Serena brings Nate to distract her and Colin brings Juliet, who is angling for an invite. Nate misinterprets Colin for Ben, which causes him to warn Juliet to be careful when it comes to her brother. After the party, Juliet installs cameras into Colin's office, which catches him kissing Serena.
In Juliet Doesn't Live Here Anymore, Colin and Serena agree to go away for the weekend. After talking with Dan and Blair, Serena realizes she's worth more than someone who has to go to extreme lengths to hide their relationship and breaks up with Colin. He quits his job at the university and surprises Serena at the ballet. While there, Juliet and Vanessa try to tell Dean Reuther about Colin and Serena but fail with no proof. After discovering Juliet took the pictures, he cuts her off completely. In the cab later that evening, Colin and Serena break up amicably and he decides to focus on business instead of teaching.
- Serena van der Woodsen (fling)