User blog comment:B214/Volume 22 spoilers/@comment-50.139.232.230-20160610153859/@comment-24.113.252.213-20160619070936

Valkyries are female warriors under the command of Freyja, and Einherjar are male warriors under the command of Odin (I'm assuming they're all male because I can't find any female Einherjar in any form of literature).

The Norse gods select humans who have proven their strenghth in battle, revive them (we know the Norse gods are capable of this because they revived Fafnir), and welcome them into Asgard to hone their skills and fight when Ragnarok comes.

But since Christianity spread, and less and less people believed the Norse religion, the Norse gods had less and less warriors to revive and add to their ranks, so they had the Valkyries and Einherjar marry and increase their forces that way.


 * This is my speculation.