User blog comment:IamJakuhoRaikoben/Chapter 480: The Northern Gravestone, Review./@comment-5303512-20160404151842/@comment-4829046-20160405121346

The reason he was kept out of the main story and only hyped up in the Vampire flashback, was to show that he is a walking deus ex machina. :P If plot demands it, his godly skills will activate and roast any demonic being in his path. His 'Dr. Jekyll' exists to keep some level of normal human left so that we can relate.

And you know why it works? Because it makes no sense for the others to survive with their level of power. Better to bring up a character that was hyped up from the very beginning as someone even Meliodas thinks is the strongest than ruin existing characters.