Charlotte is a customer at Clappy's, a restaurant that Peter works at in a cutaway gag in "Undergrounded".
The employees at Clappy's sing "Happy Birthday" to the guests by spelling out all letters, including their names, which turns out to be a long and laborious process and leaves both the employees and their guests exhausted. However, when they complete Charlotte's song, another table requests the same in tribute to Hawaiian singer Israel Kamakawiwo'ole.