Answer
Cao Shuang was a co-regent turned adversary to Sima Yi. He was the son of Cao Pi, the first emperor of the Wei dynasty, and was appointed as co-regent with Sima Yi after Cao Pi's death. However, Cao Shuang was a weak and incompetent ruler, and Sima Yi gradually gained more power and influence. Eventually, Sima Yi overthrew Cao Shuang and established the Jin dynasty.