Play as Solid Snake:
Complete 100% of the game (get all coins) with all default characters to unlock Solid Snake as a skater.
All Characters/Big Heads Code:
Up, Down, Left, Right, Up, Down, Left, Right, Up, Down, Left, Right, O.
All Stages Code:
L2, R2, Left, Right, Left, Right, Left, Right, Down, Down, Up, Up, Down, Up.