Character unlocks differs between the 3DS and Wii U versions. I'm posting a complete list for both versions below. [Source](http://ca.ign.com/wikis/super-smash-bros-wii-u-3ds/Unlockables)

Ness
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**3DS**: 10 Smash matches or beat classic mode  
**Wii U**: Unlocked by default


Falco
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**3DS**: 20 Smash matches or beat Classic mode  without using any continues   
**Wii U**: 10 Smash matches or beat Classic mode


Wario
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**3DS**: 30 Smash matches or complete 100 man smash   
**Wii U**: 20 Smash matches or complete 100 man smash

Lucina
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**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
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**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
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**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.
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**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
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**3DS**: 80 Smash matches or beat Classic mode as Link at intensity 5.0 or higher   
**Wii U**:  Unlocked by default


Mr.Game & Watch
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**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
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**3DS**: 100 Smash matches or beat Classic Mode as Bowser on Intensity 6.0 or higher   
**Wii U**:  Unlocked by default


Duck Hunt
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**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
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**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.

Mewtwo
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**3DS**: $3.99<br>
**Wii U**: $3.99<br>
**Both**: $4.99<br>

Lucas
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**3DS**: To be determined<br>
**Wii U**: To be determined<br>
**Both**: To be determined<br>