There are 12 hidden characters in Super Smash Bros. for Nintendo Wii U and 3DS. These characters can be unlocked from Classic, Trophy Collection, Equipment Collection, and Mulit-Man, and Smash.
How to Unlock Hidden Characters
Smash is one of the best methods to unlock hidden characters. You need to play a certain number of pre-requisite Smash Battles to start a one-o-one battle with a hidden character.
Win the battle with that hidden character to unlock him/her. Solo Smash, Group Smash, and Online Smash can be used to unlock these characters.
You need to play 10 matches or clear Classic on any difficulty.
You need to play 20 matches or clear Classic without using any continues.
You need to play 30 matches or complete 100-Man Smash.
You need to play 40 matches or clear Classic without any continues and using Marth.
You need to play 50 matches or clear Classic with at least three characters.
You need to play 60 matches or clear Classic on at least 4.0 intensity while using Mario.
You need to play 70 matches or collect at least 200 different trophies.
You need to play 80 matches or clear Classic on intensity 5.0 or higher while using Zelda or Link.
Mr. Game & Watch
You need to play 90 matches or clear Classic with at least 10 characters.
You need to play 100 matches or clear Classic on at least intensity 6.0 while using Bowser.
You need to play 110 matches or clear Classic with at least 15 characters.
You need to play 120 matches or collect at least 30 different equipment items.