Ryōbu Shintō emerged less as the creation of a single founder and more as the crystallization of a particular way of seeing the relationship between buddhas and kami. Within this gradual development, Kūkai (also known as Kōbō Daishi) stands out as the central architect. As the founder of the Shingon school of Esoteric Buddhism in Japan, he provided the doctrinal and ritual framework that allowed Shinto deities to be interpreted as manifestations of Buddhist divinities. His teaching on the “two realms” mandalas, and the identification of kami with the cosmic Buddha Dainichi Nyorai, gave this syncretic vision both its structure and its symbolic language. In this sense, Ryōbu Shintō can be seen as a natural flowering of Kūkai’s esoteric cosmology into the sphere of indigenous kami worship.

At the same time, this synthesis did not unfold through Kūkai alone, but through the sustained efforts of his successors and related lineages. Early Shingon monks at centers such as Tō-ji and Kōyasan elaborated the practical side of this vision, pairing specific kami with buddhas and bodhisattvas and integrating shrine worship into esoteric ritual life. Tendai clergy also contributed to this combinatory religious culture, advancing theories that framed kami as manifestations of Buddhist realities and fostering shrine–temple complexes where these ideas could be lived out. Thus, while Kūkai is remembered as the pivotal figure whose thought shaped the core of Ryōbu Shintō, the tradition itself took form through a wider community of monastics who extended and systematized his insights into a comprehensive kami–Buddha syncretism.