We recently learned that King is a member of the Lunarians, a rare thought to be extinct race of humanoids from the Red Line that can create fire.
I think Lunarians are bird people. They are completing the 3 sides of a race of people who has animal features.
1- Sea animals humans: FISH-MEN, a race of humans who have fish like appearance and control WATER.
2- Land animals humans: MINKS, a race of humans who have mammal like appearance and control ELECTRICITY.
3- Sky animals humans: LUNARIANS, a race of humans who have bird like appearance and control FIRE.
Think about it, it makes sense. Each of the 3 represent a group of animal and has power around one element: water, electricity and fire.
Now, how can King conjure fire?
Fish-Men can control the water particles who are present in the air to their advantage.
Minks use their fur to generate static electricity.
I think that Lunarians can do what the Fish-Men do to air but instead of focusing on water particles they focus on oxygen particles and can ignite them, thus creating fire.
*Theory by delgalessio