Diederich is a German nobleman holding the title of Baron, and he serves as a long-standing ally to the Phantomhive family. A baron of Weizsäcker, he is a man of considerable wealth and owns multiple castles in his homeland. In his youth, Diederich was a student at Weston College, where he was the prefect of the Green Lion dormitory. There, he strongly disliked his contemporary, Vincent Phantomhive, who was the prefect of the Sapphire Owl dormitory, due to Vincent’s relaxed and seemingly unseemly behavior. After losing a wager based on the annual cricket tournament to Vincent, his demand was for Diederich to become his fag, a subordinate underclassman, establishing a lifelong affiliation between them. During his school days and into Ciel Phantomhive’s early childhood, Diederich was a slender and athletic young man with short black hair. When he is introduced in the present day of the story, his appearance has changed considerably; he has become significantly overweight and sports a thick, handlebar mustache. Despite this, he retains his physical fitness and agility, proving to be a capable fighter.

Diederich’s personality is rough and often impatient, with Vincent Phantomhive having once described him as such. He can be rude and is known for his cautious nature, as shown when he immediately chose to relocate a private conversation rather than speak in front of an untrusted individual. He strictly adheres to rules and is intolerant of those he perceives as tardy or irresponsible. His facial expressions are frequently angry, and his demeanor is often grouchy and reluctant. However, despite this gruff exterior, Vincent asserted that Diederich is, in reality, a very kind and reliable individual who would go out of his way to aid those who need his help. He possesses great integrity, is capable of admitting defeat in a fair game, and tends to keep the promises he makes. He is also a voracious eater, with a particular fondness for sandwiches, and his gluttony has been a major contributor to his weight gain.

Diederich’s primary motivations are rooted in his loyalty to the Phantomhive family, a bond forged with Vincent that extends to Vincent’s son, Ciel. Vincent once told Diederich that if anything were to happen to him, Diederich must take care of his family in his place. This promise is a driving force for his actions. In the Emerald Witch Arc, while he complains about the trouble Ciel brings him, comparing him to his father, Diederich ultimately risks his life to fulfill this duty. His motivations for his mission in Germany are also directly tied to investigating the strange events occurring there on behalf of his allies.

In the story, Diederich serves as a crucial ally and rescuer. He attends meetings of the Aristocrats of Evil, the secret society of nobles supporting the Queen’s underworld activities, to which the Phantomhive family belongs. When Ciel becomes entangled in the affairs of the Emerald Witch and the German military, Diederich intervenes directly. He appears at a critical moment to intercept an attack from the soldier Wolfram Gelzer, who was about to kill Ciel. He then proceeds to fight Wolfram to a stalemate, using a trick to gain the upper hand before being forced to flee from a newly developed German tank. Diederich provides essential local knowledge, revealing the existence of a military railway on the eastern side of the Werewolves’ Forest that the group can use to escape. He also leads the group to the safety of one of his family’s castles, the Weizsäcker castle, where they are able to recuperate and plan their return to England.

His most significant relationship is with Vincent Phantomhive. What began as a rivalry based on mutual annoyance evolved into a deep and lasting friendship, with Vincent trusting Diederich with his family’s safety. This relationship is later mirrored in his dynamic with Ciel. Diederich often complains that Ciel brings nothing but trouble, just like his father, but his actions show a profound sense of loyalty and care for the young earl. When Ciel collapses from his injuries, Diederich shows genuine concern, and when Ciel entrusts him with a suitcase of vital evidence, stating, I trust you, it visibly affects Diederich. He also has a minor but notable interaction with the enigmatic Undertaker, who visits him in Germany, reminisces about their shared past with Vincent, and cryptically comments about the Earl of Phantomhive.

Diederich undergoes minimal personal change during the Emerald Witch Arc itself, as he is already a fully established and steadfast character. Instead, the arc reinforces his development from the brash, slender youth he once was into the reliable, if grumpy, and loyal figure he has become. His willingness to drop everything to come to the aid of Vincent’s son demonstrates how he has honored the promise made years ago, solidifying his role as a true and dependable friend of the Phantomhive family.

While known more for his strategic mind and political influence than for any extraordinary physical prowess, Diederich is notably a capable combatant. Despite his heavy build, he demonstrated he can engage in hand-to-hand combat, effectively blocking and deflecting attacks from the younger and more agile Wolfram Gelzer. He also displays proficiency with a pistol and is swift enough to react to gunfire and explosions, using his body to shield Ciel from a blast. His true abilities lie in his resourcefulness, his knowledge of German military secrets and geography, and the authority he wields as a German baron, all of which he leverages to help Ciel escape from a highly dangerous situation.