Instructions
Click on the "Story" mode and select Hitomi as your fighter. Beat the mode with Hitomi to unlock Ein.
Play through "Story" mode again with Eliot to unlock Gen Fu. Play through a third time using Zach and you'll unlock Leon.
Beat "Story" mode with the rest of the remaining characters, including Ein, Gen Fu and Leon. When you beat it with the final person, you'll unlock Helena. Play through it again with Helena to unlock Nicole, who is dressed like a Spartan from the "Halo" games.
Return to the "Dead or Alive 4" main menu and choose "Time Attack" mode. Beat this mode with every character to unlock a Tengu, the game's final hidden character.