Guts is the protagonist of the story. A tall, muscular male with a huge sword called the Dragon Slayer, a prosthetic left forearm that has a magnetic grip, and also conceals a cannon. Guts is a tragic hero; he is born as one who may be able to struggle against Causality, but seemingly unable to affect it on a large scale.

An elite Contractor with the Messier Code BK-201. Hei plays a gentle-hearted student from China named Lee Shengshun when undercover. He becomes an apathetic and guiltless killer when on his missions. Joining an organization of unknown ambitions known only as Syndicate, Hei is searching for the whereabouts of his sister Pai, whom he lost several years ago when the South American gate imploded.