Contrary to what you find in plenty of newer video games with the same Premise as Arms, the number of Downloadable Content (DLC) for this game is actually quite limited. To be quite honest, there has actually been only one major piece of DLC that was given to all players. This came in the form of Max Brass, one of the Bosses found with Arm, being given to all players through a free update as the newest playable character.
Nintendo has promised for more characters to come in the form of DLC. But as for now, players will simply have to wait for those updates to become available to them.