No special ending really. I know you do get a slightly different ending depending on how you fight the last boss, but its more of a fanservice thing. Not like booby fanservice, but REAL fanservice type nod to a certain character. Again not a huge deal.
Ein are provided randomly when you find an item, for example in my play through I found like 3 archer Einherjar which is fine cause archers > mages when it comes to combo counter. Lemme find my old threads, lots of info there.