He sees her because she's helping a drunk Serena in to a cab on Thanksgiving. He'd seen her before of course, but never gave her a second glance. She was almost always with Serena, and it was hard to see anyone else when the gorgeous, tall, blue-eyed, blonde haired girl was around. Serena was beautiful, and always seemed to attract attention to herself, and her smaller, brown-eyed, brown haired friend was forgotten. Dan didn't blame anyone-he thought Serena deserved more attention too, and he never really gave much thought to her companion. Not until that one Thanksgiving.
He doesn't see why he should be scolded by the girl because he was helping Serena-but Dan didn't have to be with the in-crowd to know Blair Waldorf could be a bitch. He thought it was another reason to prefer Serena over Blair, but then again, he didn't know the two personally yet.
There was something in her eyes when she pulled Serena away from him-something more than hatred and disgust, something he never quite figured out. But there was something about it that made him reconsider his option of Blair or Serena.
He ended up choosing Blair. He chose Serena first, just like everyone else, but in the end, it was Blair. He didn't think it was possible to surprise himself that much.