Who was Na of Wa?
Na of Wa was a pre-modern Japanese king (of the Yayoi Period) who in 57AD sent a mission and tribute to the court of Wei, mentioned in the cronicles History of the Kingdom of Wei, written in AD297. He was invested by the Han emperor as the King of Na, a kingdom in northern Kyushu. Very little is known about this king outside of his visit to the court of the Latter Han Empire.

Why is King Wa important?
The documentary evidence present in the Han histories means that the Yayoi civilizations had vessels capable of crossing the Straits of Japan into Imperial China by 57AD... a good starting date for the importation of Chinese ideas into Premodern Japan.

In 1784, a seal was discovered that corresponded to the chinese investiture of this king. It was first considered a forgery but now is deemed to be authentic.

This is a very obscure individual, one who is mentioned frequently but there is precious little detailed information in English to be used to provide more information. There may be more detailed information in Classical Chinese or Japanese, but I read neither of these languages... If you translate these languages for a living, or have more information, I would appreciate some pointers.