Genunsai is the identical twin brother of Genryusai and paternal grandfather to Kappei. He lives in the Iga region of the fictional country of Oedo. While sharing a strong physical resemblance with his brother, Genunsai possesses distinct features: two noticeable bottom teeth and wilder, uncovered hair, contrasting Genryusai's habitual hat-wearing. Genunsai's personality is markedly more energetic and unrestrained compared to Genryusai's calm reserve.

As Kappei's guardian, Genunsai plays a key role in his grandson's life. Concerned about threats seeking to exploit Gumchi, the gum-like alien entity bonded to Kappei, Genunsai sends him to train under Genryusai's tutelage at the WNA Academy in Komerika for protection. His connection to Kappei is further highlighted when an incident causes his temporary disappearance, prompting Kappei to undertake the mission "Run, Kappei! Genunsai is Missing!" to find him.

Genunsai's background is rooted in the shinobi heritage of Oedo, reflecting the series' themes of ninja lineages and their cultural significance.