The First King was an unnamed Titan Shifter who was said to have brought peace. But this peace came at a price, as most of humanity's memories were erased in the process.
Over 100 years ago, Humanity was at the brink of destruction. A Shifter known as the First King emerged, with the mysterious power of Coordination. Instead of destroying the Titans, the First King dreamed of a world in which mankind was ruled by them. He created the Walls, then created the Reiss Bloodline of Kings. The First King then eliminated anyone's knowledge of these events, in the hope of creating his perfect utopia.
The only people immune to his mental manipulation were the Ackermans the the Asian Clans. This lead to centuries of persecution to both groups, as both were nearly on the brink of extinction.