In the 3rd year of Kōgyoku-tennō's reign (皇極天皇三年), the empress abdicated; and the succession (senso) was received by her younger brother.

In 645, he ascended to the throne two days after Prince Naka no Ōe (Emperor Tenji) assassinated Soga no Iruka in the court of Kōgyoku.

Rather, it was presumably Sumeramikoto or Amenoshita Shiroshimesu Ōkimi (治天下大王), meaning "the great king who rules all under heaven".

In 653, Kōtoku sent an envoy to the court of the Tang dynasty in China, but some of the ships were lost en route.

Naka no Ōe held the rank of crown prince and was the de facto leader of the government.

The system of hasshō kyakkan (eight ministries and a hundred offices) was first established during the reign of Emperor Kōtoku.

[9] Traditionally the monk Hōdō, was born in today India, and traveled to Japan via Tang China and the Baekje kingdom in Korea.

Hōdō cured an illness of the Emperor Kōtoku (596 – 654 AD), who then sent the monk to establish numerous Buddhist temples.

Kugyō (公卿) is a collective term for the very few most powerful men attached to the court of the Emperor of Japan in pre-Meiji eras.