In the beginning of the series, Isshin is depicted as a goofy, fight-loving father who takes a strong interest in his children's activities. However, he often expresses this interest in an offensive or overly cheerful manner, which usually results in comical reactions from his kids, typically involving a kick. Isshin enjoys teasing Ichigo and frequently talks to the large memorial poster of his late wife Masaki, which he keeps in their kitchen, especially when he feels emotional.
Despite his eccentric behavior, Isshin can be serious and somber at times. On the anniversary of Masaki's death, he smokes a single cigarette by her grave because she once told him it made him look cool. This was the first and last time she complimented his appearance. He also reassures Ichigo that he shouldn't blame himself for his mother's death, as she sacrificed herself to protect him. During his confrontation with the arrancar Grand Fisher and subsequent conversation with Kisuke Urahara, Isshin remains entirely serious, except for brief arguments with Kon about parenting and another short exchange with Urahara regarding his resemblance to his son.
Isshin appears to be connected to Kisuke Urahara and has knowledge of Sousuke Aizen's alliance with the Menos. In a conversation with Urahara, he mentions encountering arrancar about ten years before the main storyline, though they were not as developed as Grand Fisher. He also reveals to Kon that he has been aware of Ichigo's shinigami activities and Kon's substitutions since the first attack by Grand Fisher. Isshin notes that he has never called Kon "Ichigo" while Kon is in Ichigo's body and that the traveling charm he gave Ichigo before rescuing Rukia was actually meant for Kon (it’s a protection talisman). This demonstrates his sensitivity and propriety as Ichigo's father, traits rarely shown in the series.
Isshin also shares a relationship with Ryuuken Ishida, similar to that of their sons. When Ryuuken finds out his son is missing, Isshin visits him, and they discuss their parenting styles. It is revealed that Ryuuken knows about Isshin's true nature and comments on his powers. The two have a history together and have been acquaintances for a long time. Both are doctors; Isshin runs a clinic, while Ryuuken manages a hospital. There are hints that they cooperate professionally, with Isshin handling minor cases and occasionally seeking help from Ryuuken.
