The Seikoku is a symbol within the world of Seikoku no Dragonar.
Long ago, the Mother Dragon created a ritual whereby all Dragons would bond themselves with a human partner. This ritual, the Orphan Rite, involved marking a young human (roughly seven years old) with what is known as the Seikoku, or Starbrand.
The Seikoku is a symbol made up of several markings that together resemble a dragon head. Within this symbol, rests an infant dragon, which will eventually be born from the markings. Afterwards, the Seikoku acts as a link between the Dragon and their rider, and allowing the flow of Astral from rider to Dragon.