Sou Sei (Chinese: Cao Zheng) is a fictional character in Water Margin, one of the Four Great Classical Novels in Chinese literature. Nicknamed "Knife Wielding Demon", he ranks 81st among the 108 Stars of Destiny and 45th among the 72 Earthly Fiends.
Sou Sei is a butcher from Dongjing (東京; present-day Kaifeng, Henan), the imperial capital of the Song Empire. Due to his excellent skills in butchery, he earns himself the nickname "Knife Wielding Demon". He also learnt martial arts from  Rin Chuu (Chinese: Lin Chong) before when the latter was still the martial arts instructor of the imperial guards.
Sou Sei becomes the chief butcher of the Suikoden Outlaws after the 108 Stars of Destiny come together in what is called the Grand Assembly. He is in charge of overseeing the slaughtering of cattle and poultry. He is killed by a poisoned arrow at the battle of Xuanzhou (宣州; present-day Xuancheng, Anhui) during the campaign against Hourou's (Chinese: Fang La) rebel forces.