If you haven't purchased all of the extra characters yet, now is a great time to grab what you're missing.
Valve have very kindly allowed us to put some character bundles together, which offer savings on anything you don't own. There is no way to 'pick and choose' which characters to buy, as there is no system for that, so we've created themed bundles as suggested on this forum a while ago.
For each bundle, you effectively get 3 for the price of 2, or 33% off. So, if you don't own any of the 3, you'll get the 3 for the usual price of 2. If you own 2 already and are just missing 1, you'll get 33% off that 1 character.
Here are the bundles -
Magical Ladies - Illusionist, Apprentice Mage and Black Witch
http://store.steampowered.com/bundle/326/
From Mysterious Lands - Genie, Goblin Shaman and Shape Shifter
http://store.steampowered.com/bundle/328/
The Tricksters - Jester, Courtesan and Gambler
http://store.steampowered.com/bundle/329/
Light and Dark - Martyr, Exorcist and Devil's Minion
http://store.steampowered.com/bundle/327/