How do you unlock a character in resident evil 5 mercenaries

Keep this in mind that I actually don't have the game so I'm not 100% sure about this but I can say this. Try beating a place with someone that you already have like Chris and try getting a 4 star ranking or whatever you need to do in order to unlock a character. The game should tell you how to unlock the characters in the Mercenaries. So, just try and do what they say.