Who or what was David Shephard?

Question

Spoilers for the final season of LOST below.

In the flash-sideways universe, Jack and Juliet have a son named David Shephard. Unlike other major characters of that world, he has no known real-world counterpart. He does not move on with the other characters at the end of the finale.

So who or what was he?

• Was he an entirely fictional construct of the shared flash-sideways reality?
• Was he some other people's son in the real-world, and "given" to Jack and Juliet to help them gain closure?
• Was he actually Jack and/or Juliet's son in the real world, somehow?
• Or was he something else?

Answer 1

David was not present in real timeline and only existed in flash-sideways universe, so we can't find his real world counterpart. There are various theories about his role/destiny.

But to me he is just a filler character used to give closure to Jack, Jack is the one who never cleared from his daddy issue, rest of the characters almost did. So David was just there to role reverse the Jack's situation and give him a closure before they move on to that spiritual plain they are going.

His name is reference to Shepard boy David. From trivia section of lostpedia.wikia.com:

Judeo-Christian tradition holds that a "shepherd" boy named David (who also was a gifted musician) was the only person in Judea brave enough to fight the giant Goliath. He won against improbable odds and became the king of Judea. The most famous Psalm of David, Number 23, opens with the line, "The Lord is my shepherd."