Lt. Col. Masataka Ida: He was in the Military Affairs Section of the Japanese Ministry of War. Ida was convinced for a time to help Hatanaka, by asking for support from Lt. Gen. Takeshi Mori of the 1st Imperial Guards Division, and from the Eastern District Army. As he explained his reasoning to Mori, he became more and more passionate about going through with the plot. However, after Mori's refusal to support the uprising, and subsequent murder, and Ida's inability to gain the support of the Eastern District Army, he decided that the plan could never succeed. He went to the Imperial Palace to warn Hatanaka that the Eastern District Army was on their way to stop him, and that he should give up. Ida felt that there was no longer a chance for success, and the only honorable end to it could come from suicide. Can we say he was a major plotter? No.
Lt. Col. Masahiko Takeshita: He was the senior man, but he was not the leader. He was the head of the domestic affairs section of the Military Affairs Bureau of the Imperial Japanese Army. He was also the brother in law of the war minister of the time, Korichi Anami. The plotters went to him asking him to intercede on their behalf to the minister. The minister refused to commit himself one way or the other, and in the end, did not help the potters at all. In the end, the cabinet decided to end the war, forcing the hand of the plotters. He supported the plotters in that he did not want to surrender to the Allies, but he refused to command soldiers to the palace. In the end, Hatanaka tried to convince him one last time, but he refused. He also knew that his brother in law and friend Anami would commit seppuku due to the fact that the military had lost the war. He went to Anami's house, and acted as his second, cutting off his head when the time came. Again, major plotter: No.
Lt. Col. Jiro Shiizaki: He was a member of the staff of the domestic affairs section of the Military Affairs Bureau's War Affairs Section. Shiizaki was one of the few to be involved in the climactic action; the rebels, with the help of the First Imperial Guard Division, seized the Imperial Palace, held Emperor Hirohito under, essentially, house arrest, and sought to destroy the recordings which had been made of the Emperor's surrender speech. Here is where it becomes foggy. According to another source, it was Shiizaki that went to the commander of the 2nd Infantry Regiment and convinced him to stay out of the plot. He was probably the most active of the Lt. Cols. But I still believe that Capt. Uehara was the real second plotter. He even assassinated two high ranking officers when they refused to help the plotters and committed seppuku himself when he realized all was lost.
Lt. Col. Masao Inaba: I think he must have been a spook or sombody's strange mentally retarded cousin or something like that. He is mentioned in many sources as a plotter, but I cannot find any other information on him. Go figure.
Anyhoooo, that's my weird, convoluted logic.
Final Answer:Major Kenji Hatanaka and..... wait for it....... Lt. Col. Jiro Shiizaki
Dean
Man, my fingers are tired.