Walt has had a nomadic existence most of his life, traveling the world with his mother on business trips. Deprived of the ability to establish roots or friends, he consequently relates to adults better than his peers. Now stranded on the island with a father he doesn't really know - - - and doesn't want to know.
Every viewer thought that Walt's role in the show would have been bigger. But the writers guide gave him the smallest description and the single mission to reject Michael as his father.
But even in that small description, there are several very LOST themes at play. Loner. Loneliness. Friendship. Daddy issues. Abandonment. Relationships.
There was nothing in the description that tied Walt's character to the inner darkness of the island, or his ability to make birds commit suicide, or see ghost images or have supernatural powers. It would seem that Walt's role would be that of a disengaged but rebellious 10 year old boy.
Perhaps that is why Walt was an after-thought "main character." The producers did not know how to handle a child in an adult environment that was going to have an adventure, i.e. danger, story engine week to week. There are only so many scared kid in a tent featurettes in a writer's quill.
It may have been foreshadowing of what was going to happen to Walt in real life. The show was going to have a linear time scope where episode to episode would invoke 24-48 hours of real island time. But since the show would only adopt a few months of run time, the real seasonal time would eclipse years. Years in which the actor playing Walt would grow up faster than the series built-in time code. Literally, Walt "out grew" LOST before LOST outgrew his character. I don't think the writers thought too far ahead with the growing up Walt problem because they put into his back story some major sci-fi elements and tied him to the island as being "special," but failed to explain why. Perhaps the ill-fated time travel loop was going to be a means of keeping Walt later into the series, but that was dreadfully executed that Walt's character never made a meaningful return. So TPTB raised Walt up to a main character, gave him significant mysteries and powers, but dropped him like a cold stone leaving fans to wonder.