In Chapter 1022 Marco says to King that he has heard rumors about a race of people who lived on the Red Line and could control fire, indicating that King was one of them and calls him King the Wildfire.
There is no information provided about the height of the Red Line, however we know that it reaches the clouds, so we can say that it is 6000 meters above sea-level, while the “White Sea” of the Sky Islands starts at 7000 meters and the “White-White Sea” starts at 10000 meters.
During the Skypiea arc, we discovered that there are two ways to get to Skypiea, one being the Knock-up Stream and the other being “The Summit of the High West”.
“The summit of the High West” could be located on the Red Line (Summit means Top and Red Line is located West of Skypiea) and since they’re just Km’s away from White Sea and White-White Sea, they could be connected with something similar to the “Milky Road”.
The King’s appearance, with fire burning, demon wings, and red eyes, makes him look like a demon. On the other hand, on Sky Island we have people who have angelic features. One of their islands is even referred to as “Angel Island”. King’s race and people of the Sky Island are the opposites.
Maybe the people from the Sky Island and King’s race waged war against each other in the past and King’s race lost it, almost drawing them to extinction, though some still live on the Red Line.
Gan Fall described using the Summit of the High West dangerous and not everyone will live using it to travel. Perhaps King’s race will kill on sight, which could explain why Bellamy survived and the rest of his crew died when travelling to the Sky Island, he is strong enough to fight them off.