Dinah Dog, also known as Koinu in Japanese, serves as one of Jack's primary animal companions, sharing residence with him, Barnaby Bear, Phineas Fox, and Squeaker Mouse. This canine regularly joins group activities, including outdoor excursions in Jack's vehicle.

During the clash with witch Auriana's forces, Dinah Dog and the other animal friends become entangled when the harpy Harvey attempts to lasso Jack. The group demonstrates collective resilience, such as driving off the young witch Allegra by pelting her with dishes after she tries to ensnare Jack with a magic rope. Dinah Dog faces peril as Auriana imprisons the companions in magical bottles, forcing them to witness Jack's struggles through a crystal ball.

Dinah Dog contributes to the resolution when Squeaker Mouse's accidental disruption of the crystal ball shatters the enchantment, freeing her and the others. Following liberation, the canine companion joins the pursuit of the witch queen and participates in the final confrontation leading to Auriana's defeat. After the witch's destruction breaks her transformation spells, Dinah Dog celebrates alongside Jack and the restored children. No additional background, development, or appearances in other media beyond the original narrative are documented.