Basically the opening question. WW2 is generally described in terms of its leaders. Hitler wanted control, he allied with Mussolini and Stalin, Churchill saw it so he teamed up with Roosevelt etc.
The exception to this is always Japan. Japan is always Japan. "Japan allied with Germany, Japan attacked Pearl Harbor, etc." There never seems to be any prominent figure associated with leading Japan's militarism and aggressive expansion. There is, of course, Emperor Hirohito, but my impression was always that he more or less remained aloof from political matters.
Japan's aggressiveness seems to have been a cultural trend that was a long time building, but surely that could be said of many nations in WWII. Who might be considered the most instrumental figure in terms of 'leading' Japan's military expansionist agenda?
You are looking for Hideki Tojo.
He was a career IJA officer who was staunchly militaristic and fascist. He also held numerous important positions in 1937 he was appointed Chief of Staff of the Kwantung Army in Manchuria, and under his watch the expansion into China and Mongolia began in earnest.
In July 1940 he was appointed Minister of War in the Cabinet(many cabinet posts were held by sitting military officers). During this time he supported the expansion of the war in China and the Tripartite Pact. Then in Oct of 1941 he was made Prime Minister, and approved the plan for war with the West and the attack on Pearl Harbor.
He remained Prime Minister and primary political leader until late 1944 when the war was decisively against Japan and following the fall of Saipan he was forced out. After the war he was tried by the War Crimes Tribunal and hung.