Isshin Arima is the paternal grandfather of the protagonist, Teppei Arima, and a central figure responsible for the dramatic shift in Teppei's life following a family tragedy. He is depicted as an elderly man with an appearance that commands respect, characterized by white hair cut to ear length, black eyes, and distinctive facial hair including a goatee and mustache. In terms of background, Isshin is the powerful and wealthy head of the Arima Financial Combine, one of the most influential industrial conglomerates in Japan. He is the maternal grandfather of Teppei, whose mother was Isshin's daughter. After Teppei's parents are killed in a fatal automobile accident, Isshin takes in his orphaned grandson, who had been living a modest life. He then adopts Teppei, giving him the Arima family name with the explicit intention of grooming him to be the heir and future successor of the vast Arima corporation. As part of this plan, he enrolls Teppei in Shuuhou Academy, an elite and prestigious private school, to prepare him for his debut into high society.

The personality of Isshin Arima is that of a commanding and pragmatic patriarch who operates with a clear vision for the future of his family's empire. His primary motivation is securing a worthy successor to take over the Arima Financial Combine, which he sets in motion by bringing Teppei into his world of immense wealth and privilege. He is also shown to be proactive in arranging Teppei's social standing, as he is the one who orchestrates an arranged marriage between Teppei and the noble and wealthy Sylvia van Hossen. While he provides his grandson with every material advantage, there is a noted point of contention between them. Isshin is not supportive of Teppei's determination to pursue revenge for the death of his parents, as Teppei believes their accident was not a coincidence but a deliberate act. This creates a fundamental conflict in their motivations, with Isshin prioritizing the stability and continuation of the family business over Teppei's personal quest for justice.

In the overarching story, Isshin Arima serves as the catalyst for the entire plot, acting as the benefactor who thrusts Teppei from a humble life into a world of opulence, political maneuvering, and aristocratic intrigue. His role is largely that of a background authority figure who sets the stage for the events to unfold but is not directly involved in the day-to-day school life of the protagonist. Beyond his relationship with Teppei, Isshin is also responsible for providing Teppei with his personal caretaker and maid, Yuu Fujikura, whom he assigns to look after his grandson and offer counsel. The character shows little in the way of personal development or change throughout the narrative, as his function is to be the immovable and wealthy patriarch whose decisions create the circumstances for the protagonist's journey. There is no notable information available regarding any specific, unique abilities possessed by Isshin Arima.