Kyousuke Kamijou
When she goes shopping with Madoka in episode one, she goes buying a disk. after which Madoka's line:"Kamijou-kun again?"
In episode two (as well as in the opening sequence), there appears an unknown silver-haired boy. In the opening, he is seen in a suit playing the violin, while when Sayaka mentions their luck in episode two, he is seen bedridden in what looks like an hospital.
Speculation
Kamijou was a famous violonist that Sayaka admires. However, for some reason, he had to be hospitalized and probably either couldn't play the violin anymore or passed away. Upon getting her wish, Sayaka then feels guilty to be so lucky and thinks about him and his misfortune. If Kamijou did pass away, then that could explain why Sayaka didn't give him her wish (yet).
Another theory states that Kamijou might be Sayaka's boyfriend (which is highly unlikely for a magical girl show) or relative. She might have bought the disc for Kamijou.