Who was the first emperor of Japan

Emperor Jimmu was the legendary first emperor of Japan according to the Nihon Shoki and Kojiki. His accession is traditionally dated as 660 BC. In Japanese mythology, he was a descendant of the sun goddess Amaterasu, through her grandson Ninigi, as well as a descendant of the storm god Susanoo. He launched a military expedition from Hyuga near the Seto Inland Sea, captured Yamato, and established this as his center of power. In modern Japan, Jimmu's legendary accession is marked as National Foundation Day on February 11.
Historians have stressed that there is no evidence for the existence of Jimmu. Most scholars agree that the traditional narrative of Japan’s founding is mythical and Jimmu is a legendary figure. In the 1930s and 1940s it was dangerous to question the existence of Jimmu. However, some details about his conquest may reflect actual events.

Wikipedia

According to mythology, Japan's first emperor, Emperor Jimmu, was a descendant of the Sun Goddess Amaterasu and enthroned in the year 660 BC. While the myths are not considered historically accurate, it is commonly accepted that emperors have reigned over Japan for more than 1500 years, and that they have all descended from the same family. The imperial crest is a 16-petaled chrysanthemum flower.

Despite the fact that the effective power of the emperors was limited or purely symbolic throughout most of Japan's history, all actual rulers, from the Fujiwara and Hojo regents to the Minamoto and Tokugawa shogun respected the emperor and were keen in having the imperial legitimization for their position as rulers.

In Shinto belief, Jimmu was a direct descendant of the sun goddess, Amaterasu.  She had a son named Ame no Oshihomimi no Mikoto who in turn had a son named Nigini-no-Mikoto. Amaterasu sent her grandson Nigini to the Japanese islands where he met and married Princess Konohana-Sakuya.  Their union produced three sons, one of whom was Hikohohodemi no Mikoto (Yamasachi-hiko) who married Princess Toyotama, the daughter of the Japanese sea god and brother of Amaterasu. The marriage of Hikohohodemi no Mikoto and Princess Toyotama produced one son named  Hikonagisa Takeugaya Fukiaezu no Mikoto. He was abandoned by his parents and raised by his mother’s younger sister, Princess Tamayori, who he later married. They had four sons; the youngest was Jimmu. From him, the lineage of all the past and present emperors of Japan can be traced.

Woodblock print of Emperor Jimmu by Ginko Adachi.

According to the Kojiki and Nihon Shoki, Emperor Jimmu  was born on  February 13, 711 BCE and died on March 11, 585 BCE. As the legend goes, Jimmu and his older brothers were born in Takachiho, the southern part of Kyūshū (modern day Miyazaki prefecture). His brother, Itsuse no Mikoto,  initially led the migration and brought the clan to the east. During his travels, Itsuse no Mikoto was killed in a battle at the hands of a local chieftain, Nagasunehiko. Jimmu took over and with the guidance of a three legged crow, Yatagarasu, reached Yamato where they again battled Nagasunehiko but won this time.

Kashihara Shrine

Jimmu’s legacy is the special bond among ruler, land, and people that made governing the land more stable. During the Tokugawa bafuku, as the military ruled Japan and the Shoguns exercised power, the Emperor was still revered and remained the unquestionable sovereign until only at the end of World War II when his divinity was renounced. In 1889, Kashihara Shrine was established in Nara Prefecture, in the spot where Emperor Jimmu acceded to the throne. It is dedicated to him.