Who is Delgadina from Memories of My Melancholy Whores and what is their importance?

Delgadina is the name of a girl in a song the narrator sings to the girl. She was the daughter of a king, and she gave her virginity to her father. The narrator likes to sing this song to the girl when they lie together at night. Over time, the narrator begins to think of the girl as Delgadina, and he even refers to her by that name when he speaks to Rosa Cabarcas.