I just rewatched season 1, and I forgot just how much mystery surrounds Walt.
1. Killing the bird at his house in Australia 2. His step dad being scared of him because he is "different" 3. Locke thinking he is so special 4. The knife in the tree ("I did like you said, I could see it...like it was real" -Walt) 5. Him being so good at backgammon 6. Telling Locke "don't open it" when he wasn't even told about the hatch yet.
I may have missed some, but these are the things I could think of off the top of my head only during season one. It seems to me they actually had plans (at this point in the series) for Walt to be an integral character involving the "white vs black" theme. I mean look at the scene of him and Locke on the beach. "One is light...one is dark", and sitting there playing the game is a "light person" and a "dark person"...on the beach...sound familiar? Like a recent scene involving Jacob and that other guy? Not really sure what the significance is yet...my point being I really think they will tie in Walt's role during the final season. After all, he kept appearing every season when he isn't really involved in any plot line. This tells me that Darlton doesn't want us to forget about him.