Sebastien Cox.
Sebastien shows up wearing what he thinks is a relatively well put-together outfit. It's something he would've worn to events back in LA, so he assumes it's appropriate. Obviously, it fails to meet Keaton's seal of approval (probably because he's wearing black sneakers).
He doesn't seem altogether comfortable with the whole party idea (especially after he gets dressed down by some kid), but he's here! There's alcohol! HE'S DOING THIS THING.