Some researchers presume that his surname, Sei (井) was the abbreviation made to be one word in Chinese style and he could be a descendant of a naturalized family in the ancient times, which was the Inohe (井上) clan or the Fujii (葛井) clan.
井の姓は中国風に一字に省略したもので、古代の帰化系氏族「井上（いのへ）氏」あるいは同様の帰化系氏族「葛井（ふじい）氏」の出身ではないかと推定する研究者もいる。