Kukai traveled to Tang China in 804 CE, part of the same embassy that carried Saicho, the future founder of Tendai, and studied there under the esoteric master Huiguo, who initiated him into the teachings and ritual system later known in Japan as Shingon, "True Word," a tradition built on mandala based visualization, mantra recitation and a body of ritual transmitted directly from teacher to initiate rather than through text alone. Returning to Japan in 806, Kukai petitioned the court for a mountain retreat suited to Shingon practice and, in 816, was granted the plateau of Mount Koya, remote and difficult to reach, which he developed as Kongobu-ji, the headquarters monastery of the new school.
Where Saicho's Tendai, based on Mount Hiei near the capital, absorbed successive reforming movements that eventually broke away from it, Shingon under Kukai remained comparatively stable and centered on Mount Koya itself, developing into one of the two great schools of Japanese esoteric Buddhism alongside Tendai's own esoteric wing.
Kukai, known posthumously by the honorific title Kobo Daishi, "the Great Master Who Spread the Dharma," is remembered in Japanese tradition not simply as Shingon's founder but as a calligrapher, engineer and cultural figure credited, likely apocryphally in most cases, with feats from irrigation projects to the invention of the kana syllabary.