Isshin Shijima serves as chairman of the major automotive company Shijima Motors and is the maternal grandfather of protagonist Kokone Morikawa. In the fantasy dream world of Heartland, he embodies the King, mirroring his real-world authority. His daughter, Ikumi Morikawa, was a talented engineer who developed innovative self-driving car software. Isshin rejected her proposals and dismissed her vision, causing a professional and personal rift that led Ikumi to leave Shijima Motors and sever family ties. Her subsequent death during a software test run left Isshin with profound regret over their estrangement.

Later, as Japan prepares for the 2020 Tokyo Olympics, Shijima Motors faces pressure to showcase advanced automotive technology. Seeking to honor Ikumi's legacy, Isshin revives her abandoned self-driving project for the Olympics procession. Unbeknownst to him, his senior advisor Watanabe employs illegal methods—including framing Ikumi's husband, Momotaro, for corporate espionage—to obtain the software, driven by ambition to control the company.

Isshin's development focuses on reconciling his past. Confronted by Kokone, he admits failing to support Ikumi and expresses regret for his role in the family's fragmentation. During the climax, he intervenes to stop Watanabe's hostile schemes, demonstrating renewed commitment to his family. The conclusion shows Isshin rebuilding relationships with Kokone and Momotaro, symbolizing his acceptance of Ikumi's innovative spirit and desire for familial unity. His portrayal underscores themes of redemption and the consequences of resisting progress.