Who was the leader of north Korea in World War 2?
From 1910 until the end of World War Two, Korea was a part of the Japanese Empire, and therefore Emperor Hirohito was its head of state. The emperor's representative during that period was the Governor General, resident in Seoul. (For a list of Governors General, see http://en.wikipedia.org/wiki/Governor-General_of_Korea.)
It was only after World War II that the Soviet Union and the United States agreed to split Korea into two zones of occupation, ostensibly in order to speed up the surrender of Japanese troops and to get them out of Korea faster.
The story goes that two US State Department officials (Bonesteel and Rusk) took out a National Geographic Magazine map and saw that the 38th parallel of latitude divided the Korean peninsula almost equally into half, while keeping the capital, Seoul, within the southern (American) zone of occupation. That is why they arbitrarily decided to make that the dividing line between Soviet and American forces.
North Korea (officially known as the Democratic People's Republic of Korea) was not formally a state until September 1948, when the foundation of that state was declared, and Kim Il Sung its president.