.Your purpose as a Valkyrie is to supply Odin with warriors to cross swords with the enemies of Asgard. To this end, you are obligated - from Chapter 2 onward - to send up at least one Einherjar per chapter that fulfills basic requirements Odin has requested through Freya.
.During the Sacred Phase between chapters, Freya will inform you of Odin's requests for the coming chapter. He might request a specific type of unit such as an archer; a character who knows or has mastered a specific skill; a character with a Hero Level greater than a certain number; a character who knows certain skill or has been trained to a certain level; or any myriad of other requests.
.During the chapter following the Sacred Phase, train as many or as few Einherjar as necessary to fulfill Odin's requests, then use the menu command to Send those Einherjar to Asgard. Note that the sent Einherjar will be removed from your party after doing so and can only be returned to it during War Battles or the Seraphic Gate.
.During the next Sacred Phase, your compliance with Odin's requests will figure prominently in your evaluation. Follow them to the letter to reap great rewards.
.Note that you cannot send an Einherjar whose Character Quest you have started but not completed.