有馬 一心

The owner of the wealthy Arima corporation and grandfather of Teppei. He adopts Teppei when his daughter (Teppei's mother) and son-in-law were killed. Having learned his lesson with his daughter, Isshin allows Teppei to do as he wishes within broad guidelines rather than what Isshin wants him to do so that Teppei does not run away like his mother did.

Isshin has an excellent memory despite his advanced age and, not having been a part of his childhood, wants to get to know Teppei better. He was an entrepreneur during World War II and made his fortune in the aftermath of the war. His company extends into many areas including automotive and metal industries.