Kazunao Kokusen, a prominent Zen master of Entsū-ji monastery in Tamashima (present-day Okayama Prefecture), served as the primary spiritual teacher to Ryōkan. Ryōkan became his disciple after encountering Kokusen during a lecture at Kōshō-ji temple, drawn by the master's demeanor and teachings to follow him back to Entsū-ji for intensive training.

Under Kokusen's guidance, Ryōkan underwent rigorous Zen practice for approximately twelve years. This culminated in Ryōkan attaining satori, prompting Kokusen to formally acknowledge this spiritual awakening by granting him inka—a certification of mastery in Zen Buddhism. Kokusen passed away the year following this transmission.

Kokusen's lineage and teachings emphasized traditional Zen practices. His death coincided with significant reform within the Sōtō school, spearheaded by Gentō Sokuchū, who sought to purge elements like kōan study. Ryōkan's subsequent departure from Entsū-ji and life as a hermit may have reflected a divergence from Gentō's reforms, indirectly underscoring Kokusen's adherence to orthodox methods. Kokusen's mentorship fundamentally shaped Ryōkan's spiritual path, poetic sensibility, and commitment to simplicity.

Portrayals consistently focus on Kokusen's role as Ryōkan's teacher without expanding on personal history beyond this dynamic.