The upcoming Killer Instinct reboot is a free-to-play game that will include just one character, the ninja Jago, at launch. Additional characters will be available for purchase a la carte. But Double Helix understands that you might want to grab the full roster without paying for each of them piecemeal. It will be offering a season-pass-like option to collect them all in one go--presumably at a discount over buying them individually.
The developer confirmed the sales model to Polygon, making the free-to-play option seem more like a demo for the full game. If you're sure you'd be satisfied only playing as a few characters, though, it probably would be more economical to go with the F2P option. So far, Double Helix and Microsoft haven't confirmed any prices, for individual characters or the whole set. It will be download-only and coming alongside the Xbox One launch.