Though Simon is one of the main characters in this novel, we know very little about his background. We meet him for the first time in chapter 1 when the choir trudges its weary way across the sand to the sound of the conch. "[O]ne of the boys flopped on his face in the sand...." That was Simon. The reaction by his fellow choristers is minimal; they simply "heaved the fallen boy onto the platform and let him lie." We know from this his fainting was not particularly uncommon.
Their leader, Jack, tells them to leave him alone. The boys do mutter a protest or two, but Jack reminds them:
"'He's always throwing a faint....He did in Gib,; and Addis; and at matins [prayers] over the precentor."
The rest of the choir reacts by sniggering, so clearly they may like him but aren't averse to making him the butt of a joke. Once he recovers, he sheepishly laughs and--though he is one of the smallest "big" boys on the island--is the first to volunteer to join Ralph on his exploration of the island.
Simon faints once more, when he meets the Lord of the Flies and has a vision where he realizes what he has hinted at all along-- the Beast is them. Ironically, it is this news which gets him killed once he recovers from his faint and locates the boys in the circle.
This is a symbolic novel, and Simon represents the emotional/mystical/spiritual/intuitive component of who we are. He is also often seen as a kind of Christ figure. Simon demonstrates this more sensitive side consistently as he pauses to pick fruit for the children, reassures Ralph that he will make it home, and feels the need for solitude as he sneaks away to his hiding place with the candlebuds and butterflies.
Simon's past has obviously shaped who he is today--we just don't have many clues as to what those events and experiences were. I wish I had more to offer; however, it's clear that what Simon is and does on the island is much more significant than what happened to him before he arrived.