To unlock Ein from Dead Or Alive 2, finish Story Mode with every character. After the final credits roll (save the game byentering and exiting the Options Menu) go into Survival Mode with Hayate and play so you can put your name in the rankings (minimum three victories). While at the ranking screen, enter EIN as the name. You will unlock Ein playable in every mode except Story Mode, and you'll also unlock a Random Character Select feature.