Final answer:
King Leopold II of Belgium founded the Congo Free State and ruled it as his personal possession.
Step-by-step explanation:
The ruler of Belgium who founded the Congo Free State was King Leopold II. In 1876, Leopold established the International African Association with the goal of civilizing the natives of Africa and providing humanitarian aid. He acquired the Congo Basin in Central Africa and ruled it as his personal possession, not on behalf of the government of Belgium. The territory was declared the Congo Free State at the Berlin Conference in 1884, essentially a royal fiefdom ruled and owned by Leopold directly.