Sigma commands the Maverick Hunters' 17th Unit, spearheading operations to terminate rogue Reploids called Mavericks. He initially operates under the directive to protect humanity and maintain order, demonstrating tactical expertise during missions like neutralizing a berserk mechaniloid. In that incident, he prioritizes mission success by destroying the mechaniloid's power generator despite a hostage Reploid being endangered, articulating the philosophy that Hunters must sometimes endanger a few to save many.

He maintains professional relationships with figures such as Dr. Cain, creator of Reploids based on Megaman X's design. During debriefings on increasing Maverick outbreaks, Sigma voices concerns about X's combat hesitation while acknowledging his potential. Dr. Cain counters that X's empathy represents a unique evolutionary trait in Reploids. Concurrently, Sigma secretly masterminds a conspiracy to destabilize society, manipulating subordinates like Storm Eagle and engineering mechaniloid attacks as diversions while eliminating witnesses at crime scenes.

His betrayal culminates at a missile base where he confronts Zero, revealing his role in framing Vile for escape and data theft. After disabling communications, Sigma attempts to assassinate Zero, confirming his transformation into the primary antagonist. This sequence establishes his shift from respected commander to adversary, marked by his physical evolution including the adoption of a signature cape.

Physically, Sigma possesses a distinctive Reploid design that later incorporates more imposing elements. His leadership style combines strategic acumen with the calculated manipulation of allies and enemies.