Answer:
THE WAITER is a foil to Keri
Explanation:
In the story "dashing the tempest", the character of the server is the ideal foil to the character Keri.
Keri is one of the focal characters of the story around whom the story advances.
The server then again is one of the sub characters and his activities are the ones which legitimately negate those of Keri
I know that you said 'opening line', but I found all of that information kind of hard to fit into a handful of sentences. As a result, what I've written might really differ from your perceptions of the story. I hope that it's not too different and that it works for you. Good luck.