Dr. Hess, also appearing as Dr. Hesse, is the primary antagonist of Saber Marionette J to X. He is initially introduced as a scientist from the nation of Xian, but his true identity is far more significant. He is revealed to be the sole survivor of the first human expedition to the planet Terra II, having arrived one hundred years prior to the events of the series aboard the exploration vessel Neo Mesopotamia. His age is sustained through the use of stasis technology and cybernetic enhancements, effectively making him a cyborg who has lived for centuries.

Dr. Hess is a deeply bitter and vengeful individual whose personality has been shaped by decades of perceived abandonment. He operates with a cold, calculating, and manipulative demeanor, viewing others as tools to achieve his single-minded goal. Despite his villainous actions, his character is driven by a profound sense of loneliness and a traumatic past. His motivation stems from the belief that Earth, his home planet, deliberately abandoned him and his expedition on Terra II. This perceived betrayal festered over a century, transforming into an all-consuming desire for revenge against all of humanity. His ultimate plan is not merely to return to Earth, but to destroy it as retribution for being left behind. To this end, he requires access to ancient star maps and wormhole data stored within the cognitive circuits of the Saber Marionettes.

Initially, Dr. Hess works in the shadows, manipulating events from behind the scenes. He is responsible for creating a clone of the former dictator Faust, using this puppet to create chaos and divert attention while Hess pursued his own secret objectives. He serves as a master strategist, pulling the strings of the Xian government official Yan and other subordinates. His primary role in the story is to create the central conflict that forces the hero Otaru and his Marionettes to leave their home in Japoness and travel to Xian, where the final arc unfolds. His most deceptive act is preying on the vulnerable emotional state of Lime, one of the main Marionettes. After Otaru leaves them, a heartbroken Lime is tricked by Dr. Hess into believing he can make her human, leading her to willingly provide him with the essential data he needs to activate his ship and begin his attack on Earth.

His key relationships are predominantly antagonistic. He is a manipulator of the Saber Marionettes, particularly Lime, whom he exploits for his own gain. He also holds a relationship of cordial convenience with Yan, though Yan ultimately serves Hess's larger scheme. In the final moments of the series, a critical development occurs: Dr. Hess learns the truth about his abandonment. He discovers that the ship meant to retrieve him was destroyed in an accident, and that Earth had not maliciously forsaken him. This revelation shatters his lifelong motivation for revenge. In a final act of redemption, as his own body begins to fail from the strain of acting as his ship's central processing unit, he acknowledges his errors. Before dying, he ensures the survival of Lime, Cherry, and Bloodberry by beaming their essences into a cloning machine, inadvertently fulfilling their wish to become human. As a scientist and cyborg, his abilities include advanced technological expertise, cybernetic enhancements that prolong his life, and the capacity to directly interface with and control his massive warship, the Neo Mesopotamia.