User blog comment:B214/Question and Answers 6/@comment-9595671-20160714212150/@comment-24.113.252.213-20160715003418

Actually, Freyja is in charge of the Valkyries. Odin is in charge of the Einherjar.

The Einherjar are male warriors who were guided to Asgard by Valkyries. They live in Valhalla, where they train to hone their skills to prepare for Ragnarok. I also think they serve as guards for the Gods, along with the Valkyrie.

But since Norse mythology has lost most of it's followers (due to the spreading of Christianity), warriors can't become Einherjar anymore. So I think the Einherjar decided to marry and have children with the Valkyries to increase their forces that way. Boys became Einherjar, and Girls became Valkyries.

Although Rossweisse did say her dream was to open a Magic School in the Underworld in order to make more Valkyries out of Female Devils. So that might put holes in my theory.