Flings and romantic relationships

Book Series
Blair Waldorf (on and off girlfriend/lover)
Serena van der Woodsen (ex-lover)
Jenny Humphrey (ex-girlfriend)
Georgina Sparks (ex-girlfriend)
Lexie (fling)
Tawny (fling)
L'Wren (fling)

TV Series
Blair Waldorf
First Relationship:
Start Up: Before Pilot (101) (They are said to have been dating since kindergarten.)
Broke Up: Victor, Victrola (107)
Reason: Blair confronts Nate about his confession to 'Serena'.
Second Relationship:
Start Up: Hi, Society (110)
Broke Up: School Lies (112)
Reason: Chuck threatens to tell Nate about their relationship if Blair...

Flings and romantic relationships

Book Series
Nate Archibald (lovers/dated)
Miles (fling)
Owen Wells (fling)
Lord Marcus Beaton-Rhodes (dated)
Jason (fling)
Erik van der Woodsen (fling)

TV Series
Nate Archibald
First Relationship:
Start Up: Before Pilot (101)
Broke Up: Victor, Victrola (107)
Reason: Blair doesn't feel that Nate loves her anymore, and he refuses to confide in her about his dad's drug problem and abuse. Not to mention the fact that he confessed feelings for and kissed "Serena" at the masquerade ball.
Second Relationship:
Start Up: Hi Society (110)
Broke Up: School Lies (112)
Reason:...