Emperor of Japan
Currently, the Emperor of Japan is the only head of state in the world with the English title of "Emperor". The Imperial House of Japan is the oldest continuing monarchical house in the world. The historical origins of the Emperors lie in the late Kofun period of the 3rd–7th centuries AD, but according to the traditional account of the "Kojiki" (finished 712) and "Nihon Shoki" (finished 720), Japan was founded in 660 BC by Emperor Jimmu, who was said to be a direct descendant of the sun-goddess Amaterasu. The current Emperor is Akihito. He acceded to the Chrysanthemum Throne upon the death of his father, Emperor Shōwa (Hirohito), in 1989. The Japanese government announced in December 2017 that Akihito will abdicate on 30 April 2019.
Chrysanthemum Throne
According to legend, the Japanese monarchy is said to have been founded in 660 BC by Emperor Jimmu; Akihito is the 125th monarch to occupy the Chrysanthemum Throne. The extant historical records only reach back to Emperor Ōjin, who is considered to have reigned into the early 4th century. 
In the 1920s, then-Crown Prince Hirohito served as regent during several years of his father's reign, when Emperor Taishō was physically unable to fulfill his duties. However, the Prince Regent lacked the symbolic powers of the throne which he could only attain after his father's death.