Never heard of him indeed.
FDR promoted several American military officers to the rank of 5 Star General and the equivalent Fleet Admiral rank with the order of their promotion determining who ranked highest. Some you will recognize but Leahy and King won’t be among them.
Fleet Admiral William D. Leahy 15 December 1944
General of the Army George Marshall 16 December 1944
Fleet Admiral Ernest King 17 December 1944
General of the Army Douglas MacArthur 18 December 1944
Fleet Admiral Chester W. Nimitz 19 December 1944
General of the Army Dwight D. Eisenhower 20 December 1944
General of the Army Henry H. Arnold 21 December 1944
Fleet Admiral William Halsey, Jr. 11 December 1945
US five-star admirals: