Main Article: Daisuke Niwa
Typical 14-year-old schoolboy with a crush on Risa Harada. On his 14th birthday, his "love gene" activates, causing him to turn into Dark Mousy whenever he feels love; even a picture can trigger the gene. Daisuke is very kind-hearted, and is always trying to stop Dark from doing crazy stunts, such as stealing art. Daisuke is very athletic, even though he doesn't seem like it; in episode one, he trips on the stairs and does a front flip back up to stop himself from falling. He and Riku Harada have crushes on each other .
Main Article: Emiko Niwa
Emiko (丹羽笑子) is Daisuke Niwa's mother. She is extremely hyperactive and does not act her age. She makes unusual clothes for Dark when he goes out to steal artwork. She is also responsible for sending out the notices to the police that tell them what, when, and where Dark's next target will be. Emiko is very much in love with her husband, Daisuke's father Kosuke Niwa (丹羽小助). She has a very peppy personality, and views Daisuke’s transformation as cute.
Kosuke Niwa (丹羽小助) is Emiko Niwa's husband. He is shy and thoughtful. He married into the family and adopted the surname. He knows a lot about magic and had been journeying often, not seeing his son for many years until he finally returned when Daisuke is 14. Kosuke proposed to Emiko by giving her a white ribbon but he did not sign it. She later found out when they were both at the train station, and Kosuke dropped a white ribbon identical to the one he had given to her before.
Daiki Niwa(丹羽大樹) is Daisuke's eccentirc grandfather and Emiko's father. Daiki was the original Dark 40 years before the series takes place. He took the task of stealing during his time as Dark as though it was his job. In his youth, Daiki had bowl-shaped, crimson hair that later becomes grey and spiked like his grandson's in old age. With the aid of his daughter, Emiko, he prepares Daisuke for his destiny. He is the last of the Niwa family to transform into Dark before Daisuke. During the anime, he randomly wanders around town gaining information about Dark's potential targets.
Main Article: Wiz
Wiz, also called With, is the family pet of the Niwa family. He is also Dark's familiar, responding to Dark's call no matter where he is and transforming into black wings and clinging onto Dark's back to form his wings. He can also perform other transformations, for example impersonating Dark or Daisuke, but isn't very adept at communicating and does not like learning new words. He hates water, can't swim, is afraid of scary movies, and hates Krad.
In the English manga translation, "With" is also known as "Wiz". Wiz also learns to talk, but says "Daisuki!" instead of "Daisuke," which can be interpreted differently from Japanese (dai means "big" and suki means "like" in Japanese, so Daisuki means "really like" or can even mean "love," which translates into the English anime as "I like you"). Hence why, in one episode of the english dub, he turns to Riku and says "I like you." and smiles; he was saying Daisuke's name in english.
Main Article: Daichi Niwa
Daichi Niwa (丹羽大地) is Daisuke's future child, presumably the child of the couple Riku and Daisuke. He is only seen in Episode 13, when Towa brings Daisuke twenty years into the future in order to retrieve the stone that will allow Dark, Saehara and Risa's souls to return to their respective bodies. Daichi stole the stone from the fountain and brought it back to Emiko and Kosuke's home, he affectionately calls Emiko his "obasan", which means Grandma. Though it is possible that Daichi is not Riku's child, it is very unlikely given the facial features and his hair color being a blend of Daisuke's and Riku's.