Tokugawa Yoshinobu, the 15th shogun of the Tokugawa shogunate, lives confined within Nijō Castle. Authorities imposed this isolation due to the perilous "Natural Heavenly Song," his singing ability capable of driving listeners to madness. This confinement fuels his profound fear of loneliness. Despite his rank, Yoshinobu experiences deep isolation and leans heavily on Naosuke Ii for both guidance and emotional support, assisting Naosuke in spreading the Heavenly Song.

Naosuke Ii's death intensifies Yoshinobu's isolation, leaving him vulnerable to possession by ancestral Tokugawa spirits. Sakamoto Ryōma rescues him, helping Yoshinobu accept Naosuke's passing. This intervention sparks a close friendship; Ryōma affectionately nicknames him "Yossie." Through Ryōma, Yoshinobu discovers music as a positive outlet, eventually playing guitar during the finale.

Outside formal shogunal duties, Yoshinobu displays a gentle, childlike nature. He enjoys shogi, tea ceremonies, and traditional Japanese sweets (wagashi), though he dislikes strawberries. As shogun, he uses the formal pronoun "Yo," but adopts the casual "Boku" in personal interactions.

In the game adaptation, Yoshinobu remains confined to Nijō Castle. The anime adaptation expands his portrayal, showing him frequently sneaking into the city to alleviate boredom and revealing a more playful, adventurous side, highlighting his curiosity and desire for connection.

His journey concludes by embracing music and friendship, moving beyond dependence on Naosuke and finding solace in his bond with Ryōma, transitioning from isolation to communal artistic expression.