Games : I found meatboy. Where's the others? (Where are the character cages hidden in Dust:AET?)

In the very first area, I've found a cage that requires four keys to pick open.

Thankfully, I didn't use any of the keys on the chests found earlier on. So, I opened it.

In it, I found Meat Boy, from Super Meat Boy.

Now, I'm at the end-boss and I haven't found a single other cage containing a 'friend'. Does anybody have a list of locations where they could be found? And what characters are avaliable? (There are 12, including Meat Boy)