Daisuke Niwa is the fourteen-year-old protagonist of his story, a seemingly ordinary middle school student who carries an extraordinary family secret. His birthday is November 11. As a member of the Niwa family, Daisuke has been trained from a young age in the arts of thievery, learning to pick locks, disarm security systems, and evade traps, although he never fully understood the purpose of this rigorous education. This training is so intense that his family sets up daily life-threatening trials for him to navigate just by going to and from school, which has honed his natural athleticism to a superhuman degree, granting him the ability to jump from heights no normal boy could manage.

In terms of personality, Daisuke is kind-hearted, gentle, timid, and somewhat clumsy, which stands in stark contrast to the confident and flamboyant phantom thief he transforms into. Despite his ordinary and often awkward demeanor, he is very perceptive and possesses a strong moral compass. His true passion lies not in stealing but in creating art, and he is a skilled painter who produces beautiful works. While he dislikes conflict and often tries to rein in his alter-ego's more reckless plans, he consistently shows empathy and kindness to everyone around him, whether they are a friend or an adversary.

Daisuke's central conflict and role in the narrative are defined by a genetic curse that has affected the Niwa men for generations. When he experiences feelings of romantic love, his DNA merges with that of the legendary phantom thief known as Dark, causing him to physically transform into this alter-ego. This transformation is initially triggered by the pain of being rejected by his crush, Risa Harada. He discovers that the only way to control this power is to focus on the image of the girl he loves, so his mother gives him a photo of Risa to induce the change on command, though it is often an unwilling process for him.

Daisuke's key relationships drive much of his personal development. He begins the story as a hopeless suitor for Risa Harada, who only has eyes for his dashing alter-ego, Dark. However, over time, he grows closer to her older twin sister, Riku Harada, and gradually falls in love with her. This shift in his affections creates a complicated dynamic, as his transformations become linked to his feelings for Riku. The most significant relationship in his life is with Dark himself. Although they constantly argue and Daisuke tries to stop Dark from performing crazy stunts, they develop a bond akin to an older and younger brother, with Dark occasionally acting as a mentor figure. Daisuke also forms a complex friendship with his classmate Satoshi Hiwatari, who is secretly his rival as the commander in charge of capturing Dark, yet the two boys grow to care for each other's safety.

Throughout the series, Daisuke undergoes significant development, moving from a boy who is uncomfortable with his heritage and transformation to someone who learns to accept it. His motivation shifts from trying to win the affection of Risa to building a genuine, honest relationship with Riku, which requires him to confront the secret of who he really is. His ultimate challenge is to be loved not just for his ordinary self, but as the vessel for Dark, a persona that Riku initially hates.

Beyond his thieving skills and athletic prowess, Daisuke's most notable ability is the physical transformation into Dark Mousy. Despite his clumsiness, his lifelong training makes him exceptionally capable as a thief, though he prefers to use these skills only when forced to by his family's legacy.