A perfect girl is the same as a regular girl but with equal parts sweet and dry vermouth (a martini typically uses dry vermouth and a Manhattan typically uses the sweet but the perfect Manhattan or perfect martini uses both).
Girl: I’m here. I got the vermouth but I didn’t know if you were making martinis or Manhattans so I picked up a bottle of each.
Boyfriend: Well aren’t you the perfect girl.
If you don't know who Percy Jackson is, sit the hell down, and read both of the series,after you're done,you'll join me on the fact that Percy Jackson is the epitome of fictional character badassery