"Ideals are precious precisely because they are unattainable." ― Karlstahl to Lena, regarding the state of the Republic.
Jérôme was a tall man with the silver hair and eyes of a Celena. He wore the standard Prussian blue uniform of the Republic. His face sported a number of battle scars, in contrast to the unscarred soldiers of the Republic, signifying he was a soldier of the old generation—before the Republic Armed Forces became complacent. His beard gave off a feeling of gentle dignity.
He is a cynic and nihilist. Jérôme recognized the evil and ignorance of the Republic, and of how the Republic had thrown away its ideals. Instead of trying to change that, he simply followed the will of the people while growing more and more jaded. He believes it is the military job to listen to the will of the citizens. However he harbours bitterness and disgust to the populace.
- Chapter 1
On May 13th RY 367, he reassigned Lena to become the commanding officer of the Spearhead Squadron. While Lena wasn't confident to be placed in an elite unit, Jérôme cautioned her that being too modest would make too many enemies. He warned her about the Undertaker, the Reaper that broke its Handlers. He gave an opportunity for Lena to turn down the offer, but she accepted. He instructed Lena to do a minimal job and for her to stop interacting with the Processors. He further lectured Lena to stop recording the number of casualties in her reports, as it was unnecessary.
- Chapter 3
On June 16th, Jérôme approached an absent-minded Lena. Lena confided in him regarding the last place Václav had taken her, back on the combat wards of the Eastern Front, and that she was glad to see the reality of the nation. He sighed and criticized Václav, stating that he too was a fool who merely looked on from the sidelines. He reprimanded Lena for her ideals that are unattainable in their current situation.
- Chapter 5
On July 28th, Lena voiced her concerns of losing two processors in her last battle to Jérôme. Jérôme became increasingly irritated with Lena's suggestion to save the lives of the Eighty-Six. Jérôme rejected Lena's proposal of setting up the Republic's interception cannons, as they were located in the middle of a minefield and they couldn't afford to endanger the Republic's soldiers in order to reload them. He told Lena to stop siding with the Eighty-Six.
He then gave a new mission to Lena, and ordered the Spearhead Squadron to immediately destroy a developing Legion base in the area. Lena did not agree with the mission, as they haven't received new replacements for all of Spearhead's fallen. However, Jérôme refuted that it would be the Spearhead Squadron would suffer in the future if they don't destroy the base. Lena relented when Jérôme pressured her by her desire to save her squad and Lena reluctantly accepted the mission.
On August 16th, Lena complained directly to Jérôme in his office. She insisted for replacements for the Spearhead. Jérôme informed her that the request for resupplies and Processor replacements had been settled. He seemed more focused on the Revolution Festival party as he relocated a bottle of wine on his desk. He suggested Lena to prepare for a dress for the Revolution Festival party and it is her duty as the daughter of the Milizé family to attend.
- Chapter 6
On September 10th, Lena approached Jérôme and implored him to retract the reconnaissance mission for the Spearhead Squadron. Jérôme was impressed with the remaining numbers of the Spearhead squadron. But Jérôme still insisted the Spearhead Squadron to be sent on a suicide mission. He said it was the will of the people, as well as in the best interest of the Republic if the Eighty-Six were to perish. He persuasively pointed out that if this is found out by other countries, they will be labelled as oppressors, and would lose both face and dignity.
On August 25th RY 368, the Legion breached the Republic during the Fall of the Gran Mur and the military’s headquarters was alerted of the final defensive line's breach. He caught Lena and demanded to know of her intentions; they parted ways on disagreeable terms. He then joined the fight against the Legion. Karlstahl was confirmed to have died, presumably in battle against the Legion.
He carried an official Republic 7.62 mm caliber rifle, a semiauto three-round burst rifle. It was well maintained; the model number on it was one digit lower than the modern alterations. It was the type that had been used when Karlstahl was still in his youth, the one he’d used to fight the Legion a decade ago.
He was a friend of Lena's late father. He would come to visit often when Lena was small and play with her, and after her father’s passing, he helped arrange the funeral, as well as support Lena and her family in various ways. He thinks she is a talented, aspiring officer and he believes Václav would be proud. He commented Lena has inherited too much of Václav's ideology. He doesn't want Lena to become like him and he wanted Lena to feel the harsh reality of the nation. He believed that Lena's hope and idealism had no place within the Republic. Lena became disillusioned with him after learning he did nothing despite being aware of the Republic's fault, resulting in her strengthened resolve to walk her chosen path. Notably, after their falling out, Lena stopped seeing him as family, referring to him by his full name and rank both outwardly and in her mind.
Karlstahl had been best friends with Václav, who had fought alongside him nine years ago as part of the now demolished Republic Armed Forces. He commented Václav was a kind man who wanted to make the Republic a better place. He was certain he was a good husband and a good father. He thinks it was a terrible shame to lose a man with a strong sense of justice from an enemy attack. However he stated Václav's ignorance and stupidity was the result of his death by being so reckless for the sake of his ideals. Furthermore he pointed out that Václav didn't realize that all he did was watching the atrocities of the persecution of the Eighty-Six, just like the rest of the Republic. He continued to censure Václav for being all talk about equality while he did nothing to truly change their situation. He correctly discerned that Václav was subconsciously feeling that only the Eighty-Six could die on the battlefield, and the fact he took Lena on the battlefield proves that.