Colonel Dan Dastun serves as the head of the Military Police in Paradigm City, holding the rank of Major before his promotion to Colonel during the course of events. He is a tall man, likely in his late forties, distinguished by a mustache and black sideburns, and is almost always seen in his full military uniform, complete with a peaked cap. Dastun is a former superior officer and commander to Roger Smith, the city's Negotiator, and maintains a professional relationship with him despite no longer being his direct commander. He remains one of the very few individuals who is aware that Roger is the pilot of the giant robot known as Big O.

Described by Roger as a hard-nosed cop, Dastun is a man of immense pride who is completely devoted to the Military Police force. He is often hot-headed and cynical, typically seen with a frown, and harbors a deep disdain for the Paradigm Corporation and its executives, particularly Alex Rosewater, resenting the public perception that his forces are merely watchdogs for the powerful corporation. Dastun is highly professional, believing that police work should be left to the police, which sometimes puts him at odds with Roger's role as a Negotiator, even as the two maintain a mutual respect and occasionally share drinks together.

The primary motivation driving Dastun is his sense of duty and his desire for the Military Police to fulfill its purpose of protecting Paradigm City. This becomes a source of inner conflict, as he frequently expresses irritation at the appearance of Big O. While this initially seems like a simple dislike of a vigilante, he eventually reveals to Roger that his frustration stems from a feeling of inadequacy; the Military Police are powerless against the frequent Megadeus attacks that ravage the city, and he feels that Big O's interventions rob his force of its reason for being. Throughout the second season, this struggle with powerlessness in a world of giant robots weighs heavily on him, as he constantly witnesses the deaths of his men and his own inability to protect the population.

Dastun's role in the story is primarily that of an authority figure and an ally to Roger, often providing assistance or becoming involved in cases that intersect with law and order. One of the most significant and mysterious arcs of his personal story involves a recurring memory of a film from his childhood, Winter Night Phantom, in which a police officer on a pier shoots a woman holding a balloon. In a haunting echo of this scene, Dastun confronts and is forced to shoot a woman named Sybil Rowan, who resembles the actress from that film, at a pier as she holds a red balloon. This event deeply blurs the line between his memory and reality, and later in the series, he discovers a movie poster advertising himself as the male lead of that same film and encounters childlike versions of himself and Sybil Rowan entering a theater to watch it.

Dastun undergoes significant development, moving from a cynical officer who despises the necessity of Big O to a man who accepts the Megadeus as a necessary protector of the city. His character arc culminates in a final act of rebellion against the Paradigm Corporation during the climactic battle between Big O and Big Fau. Placing his faith in the black Megadeus that has consistently defended the city, Dastun removes his Military Police badge, throws it away, and leads his subordinates in an act of open defiance against Alex Rosewater, supporting Roger and Big O in the final confrontation. As a normal human in a world of colossal robots, Dastun has no superhuman abilities, but he is a capable leader, a skilled investigator, and proficient with standard law enforcement tools and firearms.