Really impressed with the game overall but the grind to unlock characters fucking sucks.
Such total and utter bullshit, especially when a character you are looking forward to unlocking cheese-dicks you during the challenge bout and you have to sit back and wait for that character to pop back up at some point randomly.
Yeah...that's fucking fun Nintendo.
You don't have to wait for them to pop up again. After a battle or two they will be at the challenge door, which is a new menu option that pops up in one of the main menus ("games and more" I think) in the bottom right corner. You can rematch anyone you missed in a challenger approaching fight.