Frankie Presto is the protagonist of the book. We can assume that he is a person with a deep connection to music, as the title implies something special about his strings. He may be a complex character with his own dreams, desires and challenges. His story could include his musical achievements, his influence on others, and how he overcomes obstacles in his life, all while being associated with these so - called magic strings.
I'm not sure specifically as it depends on the details within that particular fanfiction. It could be that they are two characters who have a special relationship, perhaps a romantic one, that the author is exploring in the context of the fanfiction's universe.
Yes, Frankie Presto is fictional. It's a name that exists within the realm of imagination and creative storytelling. Maybe it's from a book, a film, or some other fictional medium.