Carl Regnitz is a caring father and hard-worker. He cares greatly about his people and works hard to sever the divisions between social classes.
Despite having achieved a high rank, Carl has never forgot about where he came from and returns to his old house from time to time to relax from his hectic work and enjoy a cup of freshly roasted coffee.
Carl gets along with both the imperial family and Giliath Osborne, with the latter considered a close confidant and friend. Unlike Giliath's more confrontational nature, however, Carl is largely considered a moderate and more amicable during political discourses with their counterparts and rivals in the Noble Faction.
When he was younger, he used to live in the Ost district of the capital with his son and his niece Elsa, who he treated as his daughter. At some point Carl introduced Elsa to Arthur, a subordinate of his, who was also the heir to a count, and the two started a relationship, which Arthur's family did not approve of. As a result, she faced harassment and death threats from the noble family, and the relationship fell apart. Overwhelmed by grief, she committed suicide. This event enraged both father and son, and they came to despise the institution of nobility ever since.
Carl joined the Reformist Faction and acted towards "wiping City Hall clean of nobles". Through hard work, he moved up in the ranks of government before finally being appointed to the rank of Governor, working to better the lives of those in the lower classes and to end the discrimination between nobles and commoners with the help of Giliath Osborne.
At some point, Carl had gotten acquainted with Prince Olivert Reise Arnor. He then agreed to serve on the board of directors at Thors Military Academy and helped to create Class VII. He was especially surprised when his own son was chosen to be a member, but allowed it nevertheless in the hopes that it would allow Machias to grow out of his prejudices towards nobility.
Carl Regnitz is first introduced meeting with Class VII at Heimdallr during the preparations for the Summer Festival in middle to late July. Prior to this, he is only mentioned in passing and in the Imperial Chronicle newspaper. When Class VII arrive at Heimdallr Central Station, Carl Regnitz is late to the meeting. But, as he shows up, he ends up surprising his son.
Carl then addresses the group and introduces himself. He welcomes Class VII to Heimdallr and suggest that they move to a better location. They head to one of the Railway Military Police's briefing rooms, and there Carl begins the orientation for their field study trip. He first starts by apologizing to the group for not having orientation in a better location and admits that it is because he is really busy. He then tries to move on to the subject of assignments and accommodations, but Machias interrupts.
Machias asks why his father is the one leading their field trip and Carl responds by saying that it is because he is also one of the board of directors at Thors Military Academy. Class VII show their shock at the fact the three of their groups family members are on the board and Carl states that it wasn't planned out that way. Rean asks if there was a secret reason for why they were gathered and Carl replies that its not his place to say.
He then tells everyone that he was the one who planned their assignments and accommodations though and then gets back to the subject. Carl then explains all of the particulars of their field trip and gives them an envelope with their respective lodging addresses and house keys in it; and then leaves after explaining a bit more about it.
During the Civil War, Carl was arrested by the Noble Alliance and imprisoned in Karel Imperial Villa. He was later rescued by the crew of the Courageous and returned to his station after the events with the Infernal Castle.
Carl learned of Ash's attempted assassination on Emperor Eugent III, only to be disgusted when Osborne attempted to frame Calvard for the assassination attempt. He publicly opposed the National Mobilization Law and attempted to rally others, only for his supporters and advocates to inexplicably abandon him. As a result of opposing the central government and it being temporarily unified, Carl was sent to Ordis to serve as its provincial leader.
The group then encounters him in the great hall alongside Towa, Priscilla, Rossweisse and Claire. Carl had expected his son to arrive, but believed he was overstepping his authority. Upon hearing his son's claim that he came to inspect a potential violation, Carl told Class VII that if they were release Towa and Priscilla, they would have to defeat Claire and Rossweisse as part of "inspection and protection". He later saw his son off, motivated to struggle to the very end after he reminded him of the promise he made to Elsa.
Upon the announcement of a ceasefire during the World War, Eugent announced that Calvard was not responsible for the assassination attempt on him as Osborne had claimed. Carl immediately returned to his position as governor of Heimdallr and an interim representative of the imperial government, recalling the army to support the Emperor's words.