User blog comment:FairyRaven/Fairy Tail Chapter 356: Tartarus arc - Prologue, Review and Commentary/@comment-202.156.93.129-20131018153339

Hmm, the daddy theory might not be dead just yet as we really have no idea what a half demon spawn would look like, after all both gray and Ultear's origins are still clouded in mystery...They could possibly be half demon which would explain Ultear's exceptional prowess or a new power up for gray to tap in to his demon side(though hopefully this is not the case...gray is a dull character :/)