While Super Smash Bros. for Wii U has a large portion of the fighter unlocked from the start, there are still eight secret characters waiting to be unlocked.
Completing either requirement as specified below will see you square off against the character in a one-on-one battle, after which, if victorious, you will unlock and add them to the roster. Good luck!
Falco
* Play 10 or more Smash matches OR clear Classic mode
Wario
* Play 20 or more Smash matches OR clear 100-Man Smash
Lucina
* Play 30 or more Smash matches OR clear Classic mode at any intensity level above 5.5
Dark Pit
* Play 40 or more Smash matches OR complete All-Star on normal difficulty or above
Dr. Mario
* Play 50 or more Smash matches OR clear Master Orders on hard or higher
R.O.B.
* Play 60 or more Smash matches OR clear more than 8 turns in Crazy Orders
Mr. Game & Watch
* Play 80 or more Smash matches OR clear Classic mode on intensity 2.0 or higher with 5 or more characters
Duck Hunt Duo
* Play 100 or more Smash matches OR KO one opponent in Cruel Smash
The easiest way to get each and every character back to back is doing 8-player smaah