Ningai 仁海 (951–1046) was the great mid-Heian Japanese Shingon master who founded the Ono-ryū 小野流, one of the two principal transmission lineages of medieval Japanese Shingon (the other being the Hirosawa-ryū 廣澤流). Born into the Miyaji 宮道 clan, he was the senior disciple of Gengō 元杲 (914–995) at Daigo-ji 醍醐寺 and received the Ono-no-shibu 小野四部 transmission from his predecessors. In 991 (Shōryaku 2) he established Mandara-ji 曼荼羅寺 in the Ono district 小野郷 of southeast Heian-kyō (the present site of Zuishin-in 隨心院), which became the headquarters of the new transmission line.
Ningai’s reputation as Rain-Sōjō 雨僧正 derives from his repeated successful performance of the praying-for-rain ritual (請雨經法) before the imperial court — he is said to have produced rain on nine separate occasions. He held senior monastic appointments throughout his long life (he died at the age of 96), serving as daisōjō 大僧正 and as confidant of three emperors. He was the principal religious counselor of Fujiwara no Michinaga 藤原道長 (966–1027) and Fujiwara no Yorimichi 藤原賴通 (992–1074), and through their patronage the Ono-ryū became the dominant Shingon transmission of mid-Heian imperial Buddhism.
His principal surviving work is the seven-fascicle KR6t0179 Xiǎoyě liù tiè — the Six Booklets of Ono, the foundational ritual manual of the lineage that bears his temple’s name.