Presently, Diedrich is considerably overweight, and sports a thick, handlebar mustache. Despite the weight gain, he retains his fitness, as he was able to engage and hold his own in combat with the younger Wolfram Gelzer, and even proves his cunning, for he managed to momentarily trick Wolfram into believing Baldroy would attack him from his left.
Diedrich consistently dresses in a traditional German military suit and black boots.
Vincent Phantomhive had once described Diedrich as "impatient." On certain occasions, Diedrich can be very rude, as indicated when he had immediately departed from the room he was in to speak privately elsewhere, after Baron Kelvin interrupted him and Vincent; despite his roughness of character, this act also serves to evidence his cautious nature.
Diedrich strictly adheres to the rules and will not hesitate to confront those who oppose it. He is intolerant of those who are tardy and irresponsible.
Notwithstanding, Diedrich is capable of admitting and accepting defeat in a fair game, even to the person he is exasperated with. He possesses great integrity, and tends to keep the promises he makes. Although Diedrich's facial expressions are usually angry, and his demeanor grouchy and reluctant, Vincent had asserted that Diedrich is, in actuality, a very kind and reliable individual, who would go out of his way to aid those who require his help.
Additionally, Diedrich is a voracious eater, and favors sandwiches in particular. His gluttonous appetite for food—though, he implies that it is the stress Ciel gives him that is to blame—has significantly contributed to his weight gain.
As a young man, Diedrich was a student at Weston College and the prefect of the Green Lion dormitory, with Alexis Leon Midford as his fag. He strongly disliked Sapphire Owl's prefect, Vincent Phantomhive, for his relaxed and unseemly behavior. After Vincent left all of the work for the school's annual cricket tournament to him, he confronted Vincent. The two decided to have a wager based on the results of the tournament, the terms being that the loser will grant one wish of the winner. Vincent won, and his request was for Diedrich to become his fag, with no time limit; this established their life-long affiliation.
Sometime later, Vincent called Diedrich back from Germany. When Diedrich complained, Vincent pointed out that since he had helped Diedrich in the Windsor case a couple of years before, Diedrich must assist him with his latest case. Before Vincent could tell him the details, however, Kelvin interrupted them. Expressing his disapproval, Diedrich curtly opted to go to another room, with Vincent following him so that they may speak in private.
When Ciel Phantomhive was little, he was fearful of Diedrich because of his intimidating facial expression. After commenting on Diedrich's kindhearted nature, Vincent told Diedrich that, if anything were to happen to him, Diedrich must take care of his family in his place. When Diedrich said that he cannot imagine Vincent dying earlier than he, Vincent replied that their fates cannot be predicted.
Black Butler Arc
Diedrich attends a meeting exclusive for the Aristocrats of Evil at Phantomhive Manor. There, he discusses with Ciel Phantomhive, Chlaus, Angelina Dalles, Lau, Azzurro Vanel, and Arthur Randall the necessity of eradicating the malicious "vermin"—a drug-dealer—among them.
Phantomhive Manor Murders Arc
Ciel Phantomhive talks with Diedrich via telephone about a matter relating to the banquet he is required to host, and the latter tells him that his bad traits remind him of Vincent. He cautions Ciel not to overdo it, and the conversation ends.
Emerald Witch Arc
Diedrich is visited by Undertaker, who tells him that he has something to do in France and decided to stop by. Undertaker notes that they have not met since Vincent Phantomhive's funeral, and that Diedrich does not visit England often, wondering out loud if it is because Vincent is no longer there or because Diedrich is too weighty. He bursts into laughter, much to Diedrich's chagrin. He, then, holds up a photo of Vincent and Diedrich during their Weston College days, remarking that humans change very quickly and that Diedrich used to be thin. He quips that Diedrich's coffin must be custom-made, offering to measure Diedrich, but Diedrich refuses. Undertaker adds that Vincent would laugh if he were to see him now. Diedrich replies that he is glad Vincent is dead then, and is surprised when Undertaker sheds tears while reflecting on Vincent's death. Undertaker puts the photo back in its place, commenting that the Earl of Phantomhive is still with them, before leaving.
When Chlaus visits Diedrich in Germany to ask for information about an unwonted disease and its questionable connection to the Werewolves' Forest, Diedrich coldly dismisses him, stating that he is busy.
Soon after, Diedrich receives a letter from Ciel, which details his expected arrival. While reaching for a sandwich, he comments to himself that Ciel brings nothing but trouble just like his father, and that Ciel should look more like his father, now that he is thirteen.
On a secret mission, Diedrich enters through the main gate, and scolds the guard for presuming he is a commoner. He conducts his research about witches and werewolves while griping about how both Vincent and Ciel are terrible at handling people. He is appalled by what he discovers.
When the true connection between Sieglinde Sullivan and Wolfsschlucht has been discovered, and Ciel is on the verge of getting killed by Wolfram Gelzer, Diedrich appears and intercepts Wolfram's attack. Diedrich proceeds to fight with Wolfram, deflecting his dagger with his own pistol to a stalemate, which he then breaks by, first, tricking Wolfram into believing that Baldroy would attack from behind, and then delivering a kick and shooting his wrist. Diedrich tries to explain to Ciel that he has stumbled upon something terrible. Before he can say more, however, a destructive blast rips through the forest, forcing Diedrich to shield Ciel. Following the blast, Baldroy and Snake rush to join them. The ground begins to shake violently, and Ciel infers that they are experiencing an earthquake, a guess which Diedrich dismisses. A Panzer Germany has developed emerges, as Diedrich has anticipated. He, then, urges them to run.
While running, Baldroy shoots the tank, but lands not a scratch. Diedrich tells him that they must focus on running, for the main cannon can annihilate them in an instant. He, then, informs Ciel that, on the eastern side of the forest, there exists a military railway used to transport the materials to build Wolfsschlucht—they might be able to escape on the train. Abruptly, Ciel collapses due to his injuries. The Panzer fires, but Sebastian manages to rescue him. Diedrich exhorts them to continue running, for another attack is coming.
Ciel orders Baldroy and Snake to go to the railway with Diedrich, and alert the way the other Phantomhive servants via a flare. He, then, tosses the suitcase filled with the samples of SuLIN, brought by Sebastian, to Diedrich, obliging him to carry it. Diedrich complains about getting stuck with the worst job, and is in the process of comparing Ciel to Vincent, but Ciel interrupts him by stating that he trusts him. Ciel, then, declares that he and Sebastian will handle the tank. Diedrich is worried, but Baldroy assures him that Ciel has a plan and that Sebastian is extraordinary.
Diedrich, Baldroy, and Snake, and Finnian reach the military railway in the eastern side of the Werewolves' Forest. Diedrich is about to open a door, but Snake warns him that multiple persons are inside. Diedrich is impressed with Snake's ability, and says that though Ciel has some eccentric servants like Vincent Phantomhive did, Vincent has different tastes. He comments that it is beneficial that there are still people operating the place for they can get the train moving right away. After giving Snake the suitcase to hold, and he and Baldroy barge in and quickly eliminate the individuals inside. Subsequently, the both of them put coal in the furnace. When Finnian, Sieglinde, Tanaka, and Mey-Rin arrive, they, bar Sieglinde, also work.
The train starts to move. After an incident ensues where Grete Hilbard is killed by Wolfram, and Wolfram is shot by Hilde Dickhaut, whom of which Sebastian quickly kills, an injured Wolfram is placed on the train, and Ciel and Sebastian climb aboard as well.
The group arrive at Weizsäcker, one of the castles owned by Diedrich's family. Diedrich orders his servant Heinrich to call a doctor and prepare a bath. When the Phantomhive servants marvel at the size of the castle, Heinrich points out that it is small compared to the main castle, leading Ciel to make a comment to Diedrich about his wealth. Later that night, Ciel and Sebastian discuss Undertaker with Diedrich, and inform him about Undertaker's involvement in the Campania incident. Diedrich says that while Undertaker told him that he is going to France, he has no idea where Undertaker is at the moment. He, then, relays to them what he and Undertaker conversed about when Undertaker visited him a while ago. He admits that when Undertaker commented that the Earl of Phantomhive is still with them, he did not pay much attention to it, because he assumed Undertaker is merely speaking nonsense like he usually does.
Heinrich interrupts Diedrich, Ciel, and Sebastian's conversation, notifying Diedrich that dinner is prepared. While they are heading to the dining room, Ciel tells Diedrich that his mission will be complete once he hands Sieglinde over to Queen Victoria, and that they will leave for England as soon as the wounded are able to travel.
At the train station, Ciel and his company prepare to leave Germany. Ciel thanks Diedrich, who tells him to not bring trouble next time. Ciel picks up on the hint that Diedrich would allow them to visit next time, and Diedrich denies that he implied anything of that sort, blushing out of embarrassment. Diedrich tells Ciel to be careful, and watches the train go alongside Heinrich.
- (To Ciel Phantomhive, referring to Azzurro Vanel) "How long are you planning to let the vermin roam free? All they do is forage for food and spread plagues."
- (To Vincent Phantomhive) "The leader is representative of the pack. It means that the people who would obey you are no better than you."
- Diedrich was nicknamed "Dee" by Vincent Phantomhive. On the other hand, Diedrich used to pejoratively call him "Mole."
Out of Universe
- According to the official Kuroshitsuji's character popularity poll, Diedrich is the twenty-fifth most popular character in the series, with 45 votes. He shares this spot with Charles Phipps, who also received 45 votes.
- Diedrich appears in the short story "With Father!", written by Yana Toboso for the Kuroshitsuji Comic Anthology Rainbow Butler 2.