Dark Mousy is one of the central figures in D.N.Angel, known as the legendary phantom thief whose existence is tied to the magical lineage of the Niwa family. His origins are deeply rooted in a specific piece of art: he is one half of the Koku Yoku, or Black Wings, a powerful and cursed artwork created long ago by the Hikari family. This original piece was split into two entities during a botched theft, with Dark representing one half and his eternal rival, Krad, the other. As a result, Dark does not have a true, independent physical form. Instead, he is a spiritual or magical being who is passed down through the male descendants of the Niwa family. For forty years before the story begins, Dark was dormant because his previous host had only a daughter, and the bond can only be inherited by a male. He reawakens within the body of the young teenager Daisuke Niwa when Daisuke turns fourteen and experiences the pangs of romantic love for the first time.

In terms of personality, Dark is the complete opposite of his young host. Where Daisuke is shy, kind-hearted, and often uncertain, Dark is flamboyant, supremely confident, and unabashedly arrogant. He is fully aware of his striking appearance, which features long violet hair and matching eyes, and he uses his good looks to his advantage, particularly when flirting with women. This charming and playful demeanor is a core part of his public persona as a thief. However, beneath this confident and sometimes frivolous surface lies a more complex and isolated individual. Because he is a transient being with no body of his own, he considers his existence to be eternal but also inherently lonely, unable to form lasting, personal connections in the same way a human can. This sense of isolation fuels his primary motivation: to eventually take over his host's body permanently so he can experience the world as a real, flesh-and-blood person. Despite his selfish goal, he is not purely malevolent, showing genuine, if possessive, care for those he considers his.

Dark's primary role in the story is that of the legendary phantom thief known to the public as Kaitou Dark. His missions are highly specific: he steals magical artworks that were created by the Hikari family, many of which are dangerous and possess a life of their own. The Niwa family, with Dark as their agent, steals these pieces to seal them away and prevent them from causing harm. His heists are elaborate affairs, often beginning with a formal, public notice of his intent, during which he effortlessly evades the police and his rival, Satoshi Hiwatari, the young commander of the task force assigned to capture him.

The most significant relationship in Dark's existence is with his host, Daisuke Niwa. The two share a body, a mind, and a sensory link, which leads to a constant, bickering rivalry. Dark frequently teases and belittles Daisuke for his clumsiness and inexperience, while Daisuke is often frustrated by Dark's flippant attitude and his tendency to take control at the most inconvenient moments. Despite this constant conflict, a deep bond forms between them, akin to that of an older and younger brother. Dark genuinely worries when Daisuke is in danger and takes precautions, such as using his familiar, a rabbit-like creature named Wiz, to form his signature black wings, because using his own true wings would cause Daisuke intense physical pain. Conversely, Daisuke is one of the few people to see Dark's vulnerable side.

His romantic attachments are another key element of his character and differ significantly between the manga and anime. In the manga, Dark initially develops feelings for Riku Harada, the older and more level-headed of the twin sisters. Later, his affections shift toward Risa, the younger twin, who is initially infatuated with him. In the anime adaptation, his romantic history is more tragic; he is revealed to have once truly loved Rika Harada, the twins' grandmother. His interactions with Risa are therefore complicated, as he is initially drawn to her because she reminds him of his lost love, though he ultimately rejects her advances. His eternal foe is Krad, his other half, who exists within Satoshi Hiwatari. Where Dark has black wings, Krad has white; where Dark is playfully confident, Krad is chillingly polite and murderous. They are locked in a seemingly endless cycle of conflict that is a direct consequence of their shared origin.

Over the course of the series, Dark undergoes subtle but important development. He begins as a being who views his host and others primarily as tools or possessions to help him achieve his own goal of obtaining a human body. However, through his shared experiences with Daisuke and his interactions with the Harada sisters, he begins to change. His initial possessiveness evolves into a more selfless form of care, and he starts to sacrifice his own chances for freedom to protect the people he has come to value. In the manga's conclusion, his character arc leads him toward a greater acceptance of coexistence with the Niwa family, moving away from his singular desire to completely take over. In the anime, his development culminates in a final, sacrificial act to defeat Krad and restore peace, showcasing a profound sense of duty and care that transcends his selfish origins.

As a phantom thief, Dark possesses a number of notable abilities. Most prominently, he can fly using his trademark black feathered wings, which he typically manifests by fusing with his familiar, Wiz. He is skilled in various forms of ancient magic, often using a single black feather as a conduit to create illusions, put people to sleep, or form defensive barriers. Beyond magic, Dark is a master of physical thievery, possessing incredible acrobatic skills, a genius-level intellect for picking locks, and an almost supernatural talent for escaping seemingly inescapable traps and situations.