Ryobu Kuji
The earliest known Japanese kuji comes from the Shingon monk Kakuban (1095-1143ADE) who was an academic of Taluan’s writings and teachings. Kakuban introduces several kuji formulas dedicated to Amida Nyôrai, in his text Gorin kuji myō himitsu shaku or Gorin kuji hishaku for short. The kuji formulas Kakuban introduces are commonly grouped under the title zokushu, and are completely unrelated to original Taoist kuji. Kakuban was followed by the monk Shinran (1173-1263ADE), founder of Jōdo Shinshū sect, who introduced several kuji formulas, also dedicated to Amida Nyôrai. Kakuban and Shinran were followed by the monk Nichiren, who founded the Nichren sect, in 1730ADE who introduced a kuji prayer from chapter 26 of the Lotus Sutra where it is uttered by the King of the North, Tammon-Ten/Bishamon-Ten (Vaisravana – Skt.). None of these Japanese kuji formulas/prayers are related to Taoist kuji in any way. Though it is interesting to note that Nichiren picked Tammon-Ten, the general of the Taoist Six Chia, to dedicate his kuji prayer to. It is also important to note that the founder of Shingon, Kōbō Daishi (Kūkai), nor the founder of Tendai, Saicho, never mentions the kuji is any of their writings. And it therefore can be safely assumed they either did not know of them, or if they did, they were considered only a minor teaching at best.
The nine syllables of ku-ji have numerous correlations and associations with Japanese esoteric Buddhism. They are associated with the gorin (five chakras), the godai (five elements), certain directions of the compass, certain colors, and numerous Tantric and esoteric deities
Read more about this topic: Kuji-in