Windward
Koʻolau means windward — the side the trade winds strike. Like kona, it is a direction that became a name, and every island has a koʻolau side.
On Oʻahu it names the eastern range and the districts behind it.
The missing half
The Koʻolau range is not a mountain chain but the back wall of a volcano whose entire eastern flank slid into the ocean in a catastrophic landslide, probably over a million years ago. The debris field lies on the sea floor for a hundred miles.
The cliffs at Nuʻuanu and along the Pali are that collapse scar, softened by erosion. It is why one side of the range is sheer and the other is a long gentle slope.