Colin Forrester is a charismatic millionaire professor at Columbia University who shared a brief mutual attraction with Serena van der Woodsen before realizing who each other are. He was also Ben Donovan and Juliet Sharp's cousin who financed her lifestyle until he learned of her intentions.
Colin first appears as a womanizer in the fourth season who catches the attention of Serena van der Woodsen. Although they spend the night together talking, they later realize that Serena is a student in one of his classes at Columbia. At first, Serena prepares to drop out of his class to be with him, but a little reverse psychology from Lily lead her to put their relationship on hold.
Both decide that they can remain friends until the class is over; but one kiss is captured by Juliet Sharp who wants to get Serena in trouble, although Colin is her cousin and also her benefactor. Colin decides to quit teaching to be with Serena but she ends things with him since she still has unresolved feelings for Dan Humphrey. Although Colin should have been angry, he instead decides that he prefers business over teaching.
- Serena van der Woodsen (fling)