Goblin King

The King of the Goblin Empire is a minor character mentioned in Every Heart a Doorway.

Every Heart a Doorway
When describing his backstory to Nancy, Kade says that he killed the Goblin King with his own sword, and was appointed his heir with his dying breath, which is what led to his exile.

Beneath the Sugar Sky
The Goblin King's death is mentioned again, and Kade says that his appointment as Prince in Waiting was what made him realize how long he'd been waiting to be seen as a boy.

Kade Bronson
The Goblin King and Kade were enemies, due to their opposing sides in the war, which caused Kade to kill him with his own sword; however, the Goblin King apparently not only had a respect for Kade, but recognized that he was really a boy, as he appointed him the Goblin Prince in Waiting as he died. Kade describes him as the only adult who'd seen him clearly.