After unlocking all the other achievements I was wondering how to unlock all the characters. I looked on the internet and I could not find nothing so I wrote an email at the developer team. Here is their answer.
You unlock characters by playing games of Unspottable, (they unlock the next time you access the Start screen).
If I remember correctly, you need to finish at least 19 games to unlock them all. (It works even if it is games in 1 point)
I tried this method and unlocked the 2 remaining characters I had left so I can confirm it works. Just start a game with 2 controllers and set the point to one. Win the game and come back to the title screen in betwen each games and you will unlock it in no time.