You get to play Bucky for free if you pledge Entree tier and above. We will send Steamkey/Mobile Code to you straight to his route. You don't have to go through the hidden route to play him (you could if you want to).
Otherwise, when the game is released, you will need to purchase all 5 characters stories (all on the same platform, no mixing allowed) AND go through test and trials before you could play his route. His route is a bonus route which we want to reward everyone who supported us. Just that backers get him easier =D