Emperor Kōtoku
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.