Mercury-go is a male Labrador Retriever who serves as the messenger dog for State Section III, also known as the Pumpkin Scissors unit. Originally holding the rank of Corporal, he was demoted to Courier Private First Class after biting an officer, an incident triggered by his strong reaction to a particularly unpleasant superior. In appearance, Mercury-go has a goofy face and a friendly demeanor around his comrades, but he possesses a persistent habit of biting people on the head and refusing to let go, which becomes a recurring source of comic relief within the unit.

Despite his mischievous streak, Mercury-go is exceptionally fast, reliable, and deeply attuned to the emotions of the soldiers around him. His sensitivity to the moods of his team allows him to respond appropriately in tense situations, and he is often treated as a valued member of the platoon rather than just a working animal. When circumstances demand it, he sheds his playful nature and reveals the disciplined instincts of a military hound, capable of disarming opponents and performing tactical actions that aid the unit in the field.

His primary role is that of a courier and mascot for the Pumpkin Scissors, carrying messages between personnel and occasionally participating in missions. He shares a close bond with the entire unit, including Second Lieutenant Alice L. Malvin, and his loyalty to the group is unwavering. While Mercury-go does not undergo significant personal development over the course of the story, his consistent presence reinforces the found-family dynamic of Section III and provides moments of levity amid the series’ serious war-relief themes. His notable abilities include high-speed travel, acute emotional perception, and combat-ready behavior when protecting his handlers.