The planner of the Pearl Harbor attack was Admiral Isoroku Yamamoto (1884-1943). (The officer in charge of carrying out the attack was Admiral Chuichi Nagumo.)
Yamamoto was firmly opposed to going to war with the US. As vice-minister of the navy and an active-duty officer, he warned the Japanese premier at the time against it, saying:
If I am told to fight... I shall run wild for the first six months... but I have utterly no confidence for the second or third year.
As if on cue, Yamamoto's fleet's worst defeat, the Battle of Midway, came almost exactly six months after the Pearl Harbor attack.
Yamamoto knew America and Americans much better than most Japanese officers of his time. He had studied at Harvard from 1919-1921, and was naval attache to the Japanese embassy in Washington from 1926-1928. He travelled extensively in the US during his time there.
In those days, both the army and navy had cabinet ministers in the government who were active duty officers. These positions were exploited to exert control over the government, primarily by the army. This is one reason the Japanese Self-Defense Forces today have an agency, with no cabinet-level representation, instead of a ministry.
In addition to opposing war with the US, Yamamoto also opposed the Japanese invasion of Manchuria and the alliance with Germany. When his government ordered him to attack the US, he was the one who insisted on the sneak attack, which he considered the only thing that might give Japan a chance against the US.
Two years into the war, the US intercepted and decoded Japanese signals that detailed the dates and route for an inspection tour Yamamoto was taking. US Army fighters were dispatched to intercept him. They shot him down on April 18, 1943.