Simon is clearly a kind, thoughtful boy as he demonstrates with his treatment of the little'uns and the way he notices things that the other boys do not. He begins to really differentiate himself from the other boys when he begins to see things that they don't. He starts to gain an understanding of what the beast really is. But because he lacks a strong physical presence and is sickly, particularly when he has his fits, the other boys cannot take him seriously and thus his understanding turns out to be useless.
After he has his final vision and understands that the beast is inside of the boys, he then travels to the top of the mountain to see that the beast they thought they saw there is simply a dead pilot. His flaw leads to his death as he stumbles out of the woods and is unable to fight back; the boys kill him in a frenzy.