Ryūichirō Isaka holds the positions of Senior Managing Director and later President at Marukawa Publishing. His life intertwines profoundly with Kaoru Asahina from childhood, when Isaka's father prevented Asahina's family's suicide after financial ruin. Witnessing Asahina's trauma, the young Isaka offered him an Amadokoro plant—a gesture symbolizing "cheer up" in flower language. This prompted Asahina's first genuine smile post-tragedy, forging Isaka's enduring protective bond toward him.

A critical misunderstanding defined their early dynamic: Isaka mistook Asahina's deep gratitude toward his father for romantic affection. This misconception led Isaka to suppress his own profound feelings for Asahina for years, believing them unreturned. Their emotional conflict culminated in a direct confrontation where Asahina clarified his lifelong love was always meant for Isaka, resolving the confusion and establishing their romantic relationship—a pivotal moment earning their storyline the title "Junjou Mistake."

Professionally, Isaka displayed exceptional editorial talent despite facing nepotism allegations linked to his family's ownership of Marukawa. His career trajectory included discovering author Akihiko Usami through an unpublished manuscript; recognizing Usami's superior writing ability prompted Isaka to abandon his own aspirations as a writer. He leveraged innate editorial skills to cultivate successful authors from obscure talent, openly embracing his family name as an asset. By the main series timeline, Isaka and Asahina share a committed ten-year relationship, their anniversary depicted in animated adaptations. Their dynamic reflects deep mutual understanding, often recognizing each other's emotions wordlessly. Isaka occasionally presents a brash exterior, though underlying insecurities surface personally. His professional demeanor includes mentoring younger editors like Ritsu Onodera, whom he nicknames "Nanahikari" referencing shared experiences of familial expectations in publishing.