How to gain unique characters:
First, download a savegame of Metal Gear Solid: Portable Ops. Then make sure you have boss rush unlocked. Then go to extreme in infinitie mission. Keep an eye out for prisoners, and some of them should be uniques. Another way is to download a save of MPO, then make a new plus save game. It will ask if you want to upload soldiers from your "old" save. IF you had a save with all uniques, than you will get everyone such as Null, Cunningham, Sigint, Raikov, Ocelot, ect.
(supplied by: cruiser34)