http://www.youtube.com/watch?v=Hfy5U...ailpage#t=335s
I know the question is more about it being her theme or not when she shows up, however For the English version of the game, entering the restaurant it does say "Lilly's Seagull Restaurant" However you also only ever have actions with her in the restaurant so is debatable.
Nymph isn't so bad of a title, I assume it uses both meanings
nymph/nimf/
Noun:
A mythological spirit of nature imagined as a beautiful maiden inhabiting rivers, woods, or other locations.
A beautiful young woman.
Liete is I guess beautiful and looks young, however she is an icarian and thus is kind of "a spirit of nature" hence the spirit stones and gaia, and being of alent/ardent is because that is where she lives and waits for the party to come. Plus her super natural powers of making clone like things.