While there are usually various ways to unlock the same characters in a Super Smash Bros. game, scattered all across its various modes, it appears that the easiest method this time around is to simply play a bunch of matches -- you unlock a new character after playing every 10 matches.
Check out the order of the unlocks below.
10 matches - Ness
20 matches - Falco
30 matches - Wario
40 matches - Lucina
50 matches - Dark Pit
60 matches - Dr. Mario
70 matches - R.O.B.
80 matches - Ganondorf
90 matches - Mr. Game & Watch
100 matches - Bowser Jr.
110 matches - Duck Hunt Dog
120 matches - Jigglypuff