Lillian “Lily” van der Woodsen is the mother of Serena and Erik. She’s described in book one as “the middle-aged version of utter coolness” – perhaps that explains the casting of Kelly Rutherford.
Lily never plays a major role in Serena’s life so far as we can tell. It seems that she is content to remain in her social status and occasional check in with Serena – more like a manager than a mother. Like Serena, she is “tall, blond, thin, and super-poised”. Perhaps the fact that we don’t know much else about Lily is telling; chances are we know about as much about her as Serena does.

[...] A Gossip Girl Primer: Lillian “Lily” van der Woodsen [...]