I have become convinced that Mask de Masculine and James are actually the same being: in Chapter 565, right after they both die, Yhwach merely commente dthat "James" had died, and that his soul was going to be part of his body; however, the power of the letter was given to Mask, not James, as he himself said. Furthermore, somewhere during his fight against Rose and Kensei (or was it Renji?k), Mask stated that the star and his fans are the same person: in the light of Thwach's statement, it stops being a metaphor.
I think the "Superstar" actually caused the Quincy named James to split in two different individuals, James and Mask. Mask takes power from James, and both will keep living until the other is still alive because they share the same soul.