Daiki Niwa is the grandfather of the protagonist, Daisuke Niwa, and the father of Emiko Niwa. He is an older man, typically appearing with short, gray or white hair, though in his youth, he had bowl-shaped crimson hair. As a member of the Niwa family, he is deeply entrenched in the secret legacy of the phantom thief Dark, having been the host for this entity forty years before the events of the story begin. His advanced age does not diminish his sharpness or his lifelong commitment to the family’s mysterious mission, and he often wanders through the city gathering information on potential targets for Dark.

Daiki possesses an eccentric personality, characteristic of the Niwa family, and is a calm and pragmatic figure who treats the role of the phantom thief with a sense of duty rather than surprise or reluctance. Unlike his grandson, who is often overwhelmed by the sudden transformations and the romantic complications they trigger, Daiki displays a workmanlike attitude toward the thefts. He is shown to have taken his role very seriously, understanding the deeper reasons why the Hikari family’s artistic creations needed to be stolen, a perspective that lends him an air of quiet authority and experience. His actions are largely motivated by a sense of family duty and the desire to prepare his grandson for the same destiny he once carried. He works alongside his daughter, Emiko, to ensure Daisuke is trained for his life as Dark’s host, setting up the elaborate trials and traps that fill the Niwa household to hone the boy’s thieving instincts.

Within the story, Daiki serves as a crucial link between the past and the present. As the previous host of Dark, he is the most knowledgeable advisor available to Daisuke, offering guidance and insight into the challenges his grandson faces. He is the connective tissue that binds the generations, using his wisdom and lingering abilities to steer the current narrative without directly interfering. A key relationship from his past involves a girl named Menou. During his time as Dark, he would regularly visit her, and after she died of an illness, her spirit continued to wait for him, eventually mistaking Daisuke for his grandfather due to their strong physical resemblance. This connection highlights a more personal and poignant side to his history as a thief.

In terms of development, Daiki has already completed his primary arc before the story begins, having lived through his tenure as Dark. What remains for him is the quiet evolution from a participant to a mentor. He demonstrates that a person can continue to be deeply involved in this magical legacy without being an active host, showing that the skills and understanding he gained remain relevant. His development is reflected in his independence; whereas Daisuke transforms into the adult Dark to use most of his powers, Daiki learned to operate on his own without transforming. He developed the remarkable and rare ability to summon and use Dark’s signature wings while still in his human form, a feat he can perform much more swiftly than his grandson, proving that mastery of the family’s power extends beyond the transformation itself.