The term is translated as "Emperor" in English but its actually 天皇 = tennō, which was itself an imported neologism, a Japanese reading of Chinese characters . . . in the distant past (eg nearly 2000 years ago), he would have been was Sumerami or Ōkimi. The first character 天 means "heaven", and the second character 皇 is the Chinese character meaning [what we translate as "Emperor". The Chinese used this in the combination 皇帝 (huang di) to refer to their emperor, but again "Emperor" is just what we call it . . . The Ming Emperor wasn't the same thing as, say, a Holy Roman Emperor, or the Emperor Napoleon.

The Chinese only occasionally use 天皇 -- to refer to the first, mythical "Heavenly Emperor" and also to the "Great Emperor" Gaozong of the Tang Dynasty.

For the Japanese, linguistic and honorific parity with China was important; they held their title as equal. The next title in Asian hierarchy is one that we translate as "King" and the kanji comparison with the word for emperor tells the story. What we translate as "King" is 王 which once had the meaning of an Axe, ceremonially associated with royal power-- you can compare with the fasces of Rome, or the rod and flail of the Pharaohs. But notice that Chinese character used in Emperor 皇 -- has that King/Axe radical on the bottom, but on top there's another character for white/shining on top . . . a "shining King/Axe" would be the symbolic content.

The Japanese used their word for "King" to refer to rulers like the King of the Ryukyu -- who paid tribute to the Chinese Emperor. The Shogun Ashikaga Yoshimitsu paid such tribute to China, and was rewarded by the Chinese with the title "King of Japan" 日本国王 -- which makes him a disgraceful figure in Japanese history.

So the term we translate as King has never been used for the Tennō, no matter how limited his actual power.

Beyond that, in many periods -- including the last few centuries-- the Emperor's power has been nominal, and his role ritualistic and religious. Early western travelers to Japan compared the role of Tenno and Shogun to "Pope and King"

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Yanabu, Akira. “THE TENNŌ SYSTEM AS THE SYMBOL OF THE CULTURE OF TRANSLATION.” Japan Review, no. 7, 1996, pp. 147–157.

Spafford, David. “Emperor and Shogun, Pope and King: The Development of Japan's Warrior Aristocracy.” Bulletin of the Detroit Institute of Arts, vol. 88, no. 1/4, 2014, pp. 10–19.