Want to know how to unlock Ein? Check out the Survival Mode Video - http://tinyurl.com/DoA3Ein
TL;DR: Play survival Mode as Hayate and, in one Run, get a score of over 1000000 and 10 Wins and enter "EIN" as your name on the High Score Table.
Ein is relatively unchanged from his original appearance in Dead or Alive 2. Compared to Hitomi, he's a little slower, but hits harder. Also like Hitomi, his stronger strikes can be easily telegraphed and countered.
Tag Time Attack is pretty much the same, only there's five rounds, and not eight, and you get a Tag partner.