Karstedt is a high-ranking archnoble of Ehrenfest who serves as the commander of the Knight's Order. He holds a significant position as a trusted confidant to both the ruling archduke, Sylvester, and the former High Bishop, Ferdinand. His status within the noble hierarchy is further established by his lineage as a cousin of both Sylvester and Ferdinand, making him a member of the archducal family’s broader circle. Prior to becoming a knight, Karstedt was himself a former lord candidate, indicating his family once had a claim to the archducal seat.

Known for a calm and reliable demeanor, Karstedt is a figure others can depend upon when situations become difficult. He is described as an upright individual who adheres strictly to discipline and principles, characteristics befitting the supreme commander of a duchy’s military forces. Despite his position and authority, he is not portrayed as infallible, as his personality is noted to be occasionally slow on the uptake in certain social or complex situations. His primary motivation stems from his unwavering loyalty to Ehrenfest and its ruling family. He recognizes threats to the duchy and takes decisive action to mitigate them, as seen when he agreed to register the extraordinarily powerful commoner girl Myne as his adopted daughter to protect her and, by extension, the duchy from the political fallout her magical potential could cause.

During the events of Part 3, Karstedt's home becomes the primary residence for Rozemyne, the protagonist, following her baptism and official adoption into the archducal family. This arrangement is central to his role in the story, as he and his wife Elvira are entrusted with teaching Rozemyne the intricate etiquettes and customs of noble life. He often appears in a background capacity, lending his authority and support to the plans orchestrated by Ferdinand and Sylvester regarding Rozemyne's future. For instance, he was party to the strategic discussions framing Rozemyne as a saint to increase her value and protection. His position also requires attendance at major political events, such as traveling with the archducal couple to the crucial Archduke Conference, which leaves the internal affairs of Ehrenfest in the hands of the younger generation.

Karstedt's most significant relationship is arguably with his distant cousin Ferdinand. As a person Ferdinand trusts, Karstedt is one of the few who can observe the High Bishop's rare displays of emotional attachment and see past his usual mask of cold calculation. During the trombe subjugation, Karstedt witnessed firsthand Ferdinand's extraordinary and protective actions towards the then-Myne, actions that defied noble conventions and revealed a deep, personal fondness. Karstedt noted how Ferdinand personally escorted her on his high beast, shared his self-brewed potions, and declared custody of her, all of which were exceptional behaviors that signaled his uncharacteristic attachment. Their relationship also has a political dimension, as Karstedt's oath as Knight Commander is understood to remove him and his direct sons from the line of succession, ensuring their unwavering loyalty to Sylvester's branch of the family.

As the Knight's Commander, Karstedt is renowned as the domain's foremost warrior, possessing the martial ability and leadership necessary to lead the duchy's military in dangerous subjugations, such as the pivotal battle of Durnberg. His authority within the military apparatus allows him to oversee knightly affairs and assign personnel. Experience in high-stakes combat and internal security is implied to be part of his skillset, as he has been involved in dealing with dangerous enemies and interrogating prisoners, tasks that can leave even a seasoned knight with an exhausted look.