Another shot.
Oh, our ill-fated attorney colleague, Dr Arthur Seyß-Inquart? But as far as I know he newer was a military leader, my dear Mr. Sceadugenga.:-?
Ahh… does the mystery guest have to be a military leader? This gentleman held the rank of Reichskommissar and certainly had several thousand troops under his command including secret police but had no power over the occupying troops where he ruled.
His role would have possibly been more of a military nature than Seyss-Inquart.
Should you think he doesn’t qualify I’ll disqualify myself.
A final clue, the puppet head of the country he ruled is far better known than him.
Josef Terboven.
Ran Norway as a Nazi civil dictator but lacked power over German troops in Norway who remained under military control.
Antagonised both Norwegians and various elements of the German leadership.
Quisling is the puppet you mentioned in your last post.
Excellent analyses, my dear Mr. Rising Sun! Please proceed – I am eager to see your personal preferentials among WW2 military leaders. 
My congratulations Rising Sun.
When the war ended he committed suicide in spectacular fashion with a large amount of explosive.
The term Quisling, of course, entered the English language as an alternative to traitor to his country.
He turned the tide against Japan and altered the course of Japan’s intended war.
Nmitz?
No.
Not even an admiral.
Richard Halsey Best
Lieutenant Commander, United States Navy?
No, but he was a Commander, USN.
Perhaps our mysterious guest is Lt. Cdr. Robert E. Dixon, who was in charge of dive bombers on the carrier Lexington in the Battle of the Coral Sea? He actually planted one of a dozen bombs that, with seven torpedoes as well, sank the ill-fated Japanese carrier Shoho.
No.
He was a commander, not lieutenant commander, but he was pivotal to the Battle of the Coral Sea, among other events.
Well then… Perhaps that notorious air group commander on the USS Enterprise, Clarence Wade McClusky Jr, who was promoted into the rank of the Air Group Commander in April of 1942.
During the Battle of Midway, while leading his air group’s bombers on June 4, 1942 he made the critical tactical decision that led to the destruction of the Japanese aircraft carriers Kaga and Akagi, thus making a vital contribution to the outcome of that pivotal battle.
On the other hand his fundamental role in the Battle of the Coral Sea is pretty questionable, because that encounter was over before USS Enterprise arrived…:-?
Another possibility is that our mysterious personality is Executive Officer of the U.S.S. LEXINGTON (CV-2), Commander Morton T. Seligman. His role in previously mentioned battle indeed was significant, and his rank is harmonized with all previously mentioned conditions.
No, not him, although the mystery man also made a vital contribution to that pivotal battle, without which McClusky would have had nothing to bomb.
On the other hand his fundamental role in the Battle of the Coral Sea is pretty questionable, because that encounter was over before USS Enterprise arrived…:-?
The mystery man wasn’t at the battles of Midway or Coral Sea. He was on land at the time, making a vastly bigger contribution than he could have as a sea going officer.
Another possibility is that our mysterious personality is Executive Officer of the U.S.S. LEXINGTON (CV-2), Commander Morton T. Seligman. His role in previously mentioned battle indeed was significant, and his rank is harmonized with all previously mentioned conditions.
Not him, either.
Commander Eric Feldt?
Sorry he’s RAN.
You might be having one of those moments where you get elements of a name you’re thinking of, but can’t pull the correct one out of the memory bank.
There was indeed a RAN commander, first name Eric, whose work in the SWPA parallelled the mystery man’s in the USN.
RAN Eric (being RN Eric serving with the RAN) is unknown even in many Australian military history circles. His USN mystery man counterpart is much better known but, as this question demonstrates, still generally overlooked.
Neither had anything to do with coastwatching, but they were both watchers of a sort and doing the same sort of work. And both deposed by rivals in the USN who, in RAN Eric’s case, wanted him returned to the RN.
Thank you very much for that key-sentence …Neither had anything to do with coastwatching, but they were both watchers of a sort…, my dear Mr. Rising Sun. That was completely sufficient for my internal lamps. Our special guest was brilliant American naval cryptanalyst, captain Joseph John Rochefort. Bravo – that was indeed intriguing task. Besides, I am deeply rejoiced that we do share strong inclination toward intellectual achievements in war. 
If I was right, I think that I will have certain… crypto-mathematical surprises for you. You see, actually there was someone else in this world of unobserved masterminds, who was much, much more fundamental for the Allied war effort… 
Correct.
And his reward was dismissal. http://findarticles.com/p/articles/mi_qa3926/is_200007/ai_n8906181/pg_1
His RAN counterpart was Eric Nave. http://www.gould.com.au/Man-of-Intelligence-Eric-Nave-p/rsn012.htm
Your turn.