Cognomen せい (せい) , all around the world, is an extremely rare surname.
Cheng, Chéng, Hoshi, Jo, Joh, Jou, Jō, Mori, Sakae, Sakai, Sakan, Sakari, Sei, Sheng, Shigeru, Shing, Sho, Shoh, Shou, Shèng, Shō, Sigeru, Syo, Syoh, Syou, Tcheng, Thành, Šeng, Šó, Мори, Сакаи, Сакан, Сакари, Сигэру, Чэн, Шэн, さかい, しげる, じょう, シェン, セイ, チェン, 成, 成氏, 星, 淸, 清, 盛, 誠, 诚, and 诚氏