I can't help but think they have taken the "Manic Pixie Dram Girl" and applied her to selling burgers and fries.
You know what I'm talking about? It's that stock movie character in films like Eternal Sunshine for the Spotless Mind and Elizabethtown that exist to cheer up the often depressed male lead. Such a character is never implied to have a job or any direction in life, but just floats around the place all bubbly and random. She only exists in fantasy of course, if you encountered one of these in the real world you'd probably fear for your life and give her a little pocket change to go away, or something like that.
Heck here's the TvTropes article:
I have to wonder now if the Wendy's girl has a job, or if she actually some Wendy's-obsessed crazy nut that lives inside a Wendy's, perhaps sleeps in the dry stock area maybe?