Instructions
Acquire cash and level up enough to purchase multiplayer characters. Play the single-player mode and complete the various medal tasks, unlock trophies and discover treasure caches to build up money. If you have a completed save from "Uncharted 1" on your PS3, you automatically get an extra $80,000. In every multiplayer mode, winning gains you levels and cash.
Complete the game on "Crushing" mode (the highest difficulty setting in the game) and/or get the Platinum Trophy to unlock the last two multiplayer characters, Marco Polo and Genghis Khan, who are costumed skeletons. These are the only multiplayer characters you can't unlock with cash or by leveling up.
Purchase the special multiplayer characters on the PSN. Some of these characters, or "skins" from other PlayStation game series, come in the "Uncharted 2" expansion pack called "The Siege," which also includes new multiplayer modes and maps. Helghan from Killzone and Cole from InFamous are examples of skins.