Seki Ichinomiya directs the Ichinomiya company and fathers Kou Ichinomiya. His strict demeanor manifests as looking down on Kou, though he genuinely loves his son. He ingrained the family principle of never incurring debts in Kou from childhood, fundamentally shaping Kou's actions and reactions throughout the narrative.

Physically, Seki stands 175 cm tall, weighs 59 kg, and possesses a Libra zodiac sign with blood type A. He first appears narratively in Episode 8. His reactions to Nino evoke memories of his late wife, revealing an emotional layer tied to loss or memory.