Daiki Niwa is the grandfather of the protagonist, Daisuke Niwa, and the father of Emiko Niwa. Forty years before the start of the story, he was the last member of the Niwa family to be possessed by the phantom thief Dark, making him Daisuke's immediate predecessor in that role. Daiki has an eccentric and somewhat wandering personality, often seen traveling around town to gather information related to Dark's potential targets. Together with his daughter Emiko, he plays an active role in preparing young Daisuke for the destiny that awaits him, training him and offering guidance after the boy begins transforming into Dark.

During his own time as Dark's host, Daiki approached the required thefts of the Hikari family's magical creations with a serious sense of duty, fully understanding why these missions were necessary. He appears to have had a more even and balanced partnership with the phantom thief than Daisuke does, as he was able to work independently for long periods without needing to transform. He also gained the ability to manifest Dark's signature black wings while in his human form much sooner than his grandson did. While active as a thief, Daiki regularly visited a young girl named Menou. She later died from illness, but her spirit continued to wait for him for decades, eventually mistaking Daisuke for his grandfather due to their strong physical resemblance. Even in his old age, Daiki retains some of the talents he developed during his years connected to Dark; he is capable of flawlessly disguising himself as the legendary thief, a skill he uses to take Dark's place on a date long enough for the real Dark to arrive.