Ok, I beat the game on normal (with an A rank on only the first mission), and have every character but one.
As a note, to get some of the characters, you have to play multiplayer. I got 1 character after about 10 minutes of multiplayer, another after 25-30, and another after about an hour. You might be able to get them through single player, but I don't know.
Also, all characters I unlocked (except for multiplayer-earned ones) are only playable in survival mode.
Is it possible to get more than 10 characters in the other modes (deathmatch, blast happy and sniper battle)??
Either way, this game's multiplayer is extremely disappointing, especially compared to the original Winback - why do bots get their own screen? they count as real players? and no bosses in team-battles??? It's a shame.