Dante 01

Deep in space, above a fiery planet named Dante, orbits the space station called "Dante 01". It hosts a psychiatric detention center that currently hosts six prisoners whose crimes were so severe that they volunteered to live under experimentation. The center is owned by a pharmaceutical company called Neurinos and directed by a man named Charon, with the assistance of psychiatrist Dr. PersÃ©phone and two security guards called CR and BR.
One day, a Neurinos shuttle arrives with a new scientist named Elisa, carrying a new prisoner of unknown origins who was found as the sole survivor in a shipwreck and wakes up from the hypersleep severely disoriented and unable to speak. He is introduced to the other inmates: their leader CÃ©sar; his right-hand man Lazare, who developed the ability to hold his breath to escape the gas with which the prisoners are sedated; the large and violent Moloch, who's CÃ©sar's enforcer; the reclusive Bouddha, who has the habit of trying and murdering whoever is in pain out of misguided compassion; the deranged anarchist hacker Attila; and Raspoutine, a deeply religious man who claims the new one was sent by God to save them, and starts calling him "Saint-Georges, the Dragonslayer", because of a tattoo on his arm.
Elisa and PersÃ©phone are at odds over their research methods, since the latter advocates a humanist approach to the prisoners' mental issues, while Elisa has been sent there to experiment with a new nanotechnology that directly modifies the patient's DNA, eliminating the genetic component of their antisocial behaviors. Charon asks Attila to hack into Elisa's files to find out the extent of her orders. Much to his horror, Attila finds out she's authorized to kill them all for the sake of her research.
As expected, Bouddha attempts to strangle Saint-Georges in order to alleviate his suffering. This leads to infighting amongst the prisoners, so they get gassed, and Bouddha becomes Elisa's first subject. She injects him with nanites and returns him to the prison hold. Bouddha is immediately wracked by unbearable pain from the nanites bonding with his DNA, but Saint-Georges is somehow able to remove them from him (from his point of view, he's removing some kind of energy creature from Bouddha and eating it), so Bouddha becomes well again and as convinced as Raspoutine that Saint-Georges is a miracle worker, so he befriends him, swearing to protect him from the others. Later, Moloch attacks Saint-Georges, but Raspoutine inserts himself, causing Moloch to accidentally slit his own throat with the shiv he was holding. Gas pours in to knock the group out again, but not before Saint-Georges had run over to assist Moloch. The prison guards rush in only to discover, to their amazement, that Moloch has no wound on his throat, and is alive and well. On their way out, they inject CÃ©sar with the nanites.
PersÃ©phone's assumption that Saint-Georges is a miracle worker is scoffed at by Elisa, but Moloch's own doubts prompt an increasingly debilitated CÃ©sar to fear an impending usurping of his authority, so he commands Lazare and Moloch to get rid of Saint-Georges. While Bouddha and Raspoutine are distracted, Saint-Georges is stabbed to death by Lazare. However, while the staff is examining his corpse, Saint-Georges wakes up completely healed, grabs Elisa and uses her as a hostage to reach the prison quarters before releasing her.
CÃ©sar is now in severe pain from the nanites, but Saint-Georges cures him as well. Attila appears and tells the others of his plans to destroy them all out of spite, by making the space station crash into the planet, triggering the rage of the other prisoners. Attila manages to runs away, only to be found later in a shaft, having hanged himself.
With Attila dead, the only way to save the station is to activate the manual override located under a trapdoor below the prisoner's quarters. The prisoners agree to let the staff through, but Elisa refuses to accompany the others inside the prisoner area. As all the station inhabitants work together to open the hatch, the gas goes off, knocking everyone but Lazare out.
Elisa prepares to escape via shuttle, but Lazare ambushes her, forcing her to let him in the shuttle as well. As the others awaken, one of the guard finds out about Elisa's betrayal, but also that Attila had messed with the shuttle controls as well, resulting in the shuttle burning through the planet's atmosphere. After having finally opened the hatch, the group finds the cooling system overheated so now the corridor that leads to the manual controls is filled with boiling water. CÃ©sar realizes he is the only one short enough to fit into the little space, and volunteers to descend, wrapped in some protective fabrics. He emerges on the other side horribly burned and dies in the compartment before being able to entering the code necessary to reset the controls.
In the epilogue, PersÃ©phone's narration recounts how Saint-Georges fulfilled his destiny by slaying the "dragon". We see him emerge from the station in a space suit and float above the planet, using the full extent of his inexplicable powers and the mysterious energy creatures that now reside inside him to terraform Dante into a livable planet, while his body disintegrates in space as a result of the transformation.

Question: What is Cesar in pain from?
Answer: nanites