Dan Dastun is a major, later colonel, of the Military Police. In "Winter Night Phantom", Roger describes him as "a hard-nosed cop [...] completely devoted to the force, and he has more pride in the Military Police than anything else." He continues, "Paradigm City needs him as much as it needs [The Negotiator]." Dastun appears to be among the senior ranks as the only other officers shown in the anime are a few majors, colonels, and 1 general.
Dastun resents having the force called the "watchdogs" of the Paradigm Corporation, and has expressed his disdain for its executives. He also knows that Alex Rosewater does not care about many of the residents.
He is Roger's former commander, but they still maintain contact. Dastun's pride as a member of the military police puts him somewhat at odds with Roger after he resigns from the force, and he expresses an extreme amount of irritation every time Big O appears. At first this irritation is passed off as disliking the vigilante nature of the Megadeus. However, as the series progresses, he reveals to Roger that his irritation stems from the fact that the military police do not have the ability to protect the city adequately due to frequent megadeus attacks. This requires Big O to step in fairly often to counter these attacks, which Dastun feels robs the military police of purpose. However, he comes to accept this during season 2. He is also one of the few characters who know of Roger's affiliation with Big O, and the only such character within the military police. He was revealed to have obtained this knowledge at some point before season 2, but is only seen directly discussing it with Roger once.
Dastun remembers a memory in "Winter Night Phantom", of a film he saw as a child. In the movie, a police officer is arguing with a woman holding a balloon on a dock, ending with him shooting her. Her dying words are "Vous-êtes si gentil" ("You are so kind"). In the present, Dastun began seeing a woman that looked exactly like the actress from the movie, Sybil Rowan, roaming Paradigm City setting off explosions to assassinate the former senators. This culminated in Dastun shooting her on a pier while she was holding a balloon, living out the scene from the film. Towards the end of season 2, Dastun sees 2 children entering a theater to see the movie, the poster of which bills him as appearing, and when he follows to investigate, he finds they are child versions of himself and Sybil Rowan.