User blog comment:FlamingDragon/Recent Fairy Tail thoughts - Chapter 366/@comment-1359322-20140112090136

List of theories for Gray-Silver relationship:
 * 1) Silver is Gray's father.
 * 2) Silver is Ultear's father.
 * 3) Silver is Gray's brother
 * 4) Silver is Gray's future self.
 * 5) Silver is Gray's "darkness".
 * 6) Gray is Silver's clone.
 * 7) Silver is Gray's demon counterpart.
 * 8) Silver is Gray's Ancestor.

Place your bets.