At the Einherjar shop, you can purchase extra units to use. Some are direct copies of your own characters! The more powerful the unit is, the more expensive they are. Einherjar units cannot build support with any characters.
Einherjar units can be obtained through a few different methods. The first is when you beat the game. You may select five of your units in your army in that file that will then be in the Einherjar shop to buy. Better, more powerful units will cost you a pretty penny so think before you spend. The second method is through Bond units. If you exchange accessories with someone through visiting MyCastle three times, you will be able to buy one of their units at the Einherjar shop.
The final way to obtain Einherjar units is to defeat other players’ MyCastle armies. Instead of purchasing a skill, choose to recruit a unit.