Character unlocks differs between the 3DS and Wii U versions. I'm posting a complete list for both versions below. Source
Ness
3DS: 10 Smash matches or beat classic mode
Wii U: Unlocked by default
Falco
3DS: 20 Smash matches or beat Classic mode without using any continues
Wii U: 10 Smash matches or beat Classic mode
Wario
3DS: 30 Smash matches or complete 100 man smash
Wii U: 20 Smash matches or complete 100 man smash
Lucina
3DS: 40 Smash matches or beat classic mode as Martha without using any continues
Wii U: 30 Smash matches or beat Master Core (classic mode final boss at intensity 5.1 or greater) as Marth at 5.5 intensity
Dark Pit
3DS: 50 Smash matches or beat classic mode with three or more characters
Wii U: 40 Smash matches or beat All-Star mode on Normal
Dr Mario
3DS: 60 Smash matches or beat Classic mode as mario at intensity 4.0 or greater
Wii U: 50 Smash matches or beat one Master Order on hard difficulty
R.O.B.
3DS: 70 Smash matches or collect 200 trophies then play a Smash match or beat classic mode
Wii U: 60 Smash matches or defeat Crazy Hand after completing 10 matches in Crazy Orders
Ganondorf
3DS: 80 Smash matches or beat Classic mode as Link at intensity 5.0 or higher
Wii U: Unlocked by default
Mr.Game & Watch
3DS: 90 Smash matches or beat Classic mode with ten fighters
Wii U: 80 Smash matches or beat Classic mode with five fighters
Bowser Jr
3DS: 100 Smash matches or beat Classic Mode as Bowser on Intensity 6.0 or higher
Wii U: Unlocked by default
Duck Hunt
3DS: 110 Smash matches or beat Classic Mode with 8 or more characters
Wii U: 100 Smash matches or KO one fighter in Cruel Smash
Jigglypuff
3DS: 120 Smash matches or collect 30 different equipment items
Wii U: Unlocked by default
Additionally, Super Smash Bros for Wii U and 3DS also allows you to purchase other characters from the eShop as DLC.
DLC Characters
Mewtwo
3DS: $3.99
Wii U: $3.99
Both: $4.99
Lucas
3DS: $3.99
Wii U: $3.99
Both: $4.99
Roy
3DS: $3.99
Wii U: $3.99
Both: $4.99
Ryu
3DS: $5.99
Wii U: $5.99
Both: $6.99
(Note: This DLC comes with a stage as well as the fighter)
