Emperor Kôtoku was the 36th emperor of Japan according to the traditional counting. He was also called "Taika," for "Great Change." He ruled from 645 AD - 654 AD. He made the Taika Reforms, which changed parts of Tang dynasty ways of thinking about politics, running the government, and doing rituals for the Japanese court.
Before he was the Emperor, he was called Prince Karu. He became emperor in 645 AD after his sister, Empress Kôgyoku, abdicated in his favor. That means she quit being Empress and gave Karu the throne. Some historians say that Empress Kôgyoku's son, Prince Naka no Oe, asked her to abdicate. Prince Naka no Oe who would later become Emperor Tenji.
With the Taika Reforms, Kôtoku changed the way court officers wore clothes in 647. People of different rank (level of importance) wore different clothes. Kôtoku also said that the government had a monopoly on mined iron ore, meaning that no one else was allowed to mine iron ore. Before that, the Wakebe clan had mined iron ore.
After Kôtoku died in 654, Kôgyoku became the Empress again, but this time she was called Empress Saimei.