Of all the spirits, the Spirit King is the strongest spirit of its element.

Calling upon a King drains one of mana, in return, their mana becomes apart of one's own.[1]

Once you have made a contract with the king, you can summon all other spirits of that element without the need to form contracts with them.

List of Spirit Kings:

Sildran (Air)

  1. Volume 5, Chapter 6, page 13; Series Chapter ??; volume page 143