|I wish to make certain of something, Mr. Jaeger. As a soldier, can you continue to serve humanity by controlling your Titan ability?|
|— Dhalis to Eren Jaeger|
Zachary has the typical appearance of an old man. His eyes are sunken and tired-looking, he wears glasses, he has noticeable wrinkles on his face, and his hair is receded and gray. He also has a gray scruffy looking full beard and mustache. In addition, he is rather short which gives him a presence uncommon for a man of his rank. Zachary's casual attire consists of a plain white shirt with normal dark colored pants. In a more formal setting he adds a bolo tie, much like the three commanders under him, Nile Dawk, Erwin Smith, and Dot Pyxis. Over this he wears a brown trench coat that has a black shield emblem with a white boarder and cross.
When he was first introduced, Zachary was said to be a strict military leader who was only concerned with the best interests of humanity. As the head of the entire military, his priorities were said to lie within what is best for all of humanity. Only after being logically convinced of Eren's usefulness did he pardon him. At this time he appeared to take most things lightly, as if he had experienced them previously. During the trial of Eren he wore simple clothes and conducted the trial without any drama or passion. This indicates that Zachary has no time for drama and is solely focused on carrying out his judgments. He remains ever serious but at the same time appears tired as if his age is catching up to him.
However, after Zachary assisted in the revolution against the monarchy he shows a completely different personality. He noted to Erwin that his only interest in participating was due to an intense and long-standing hatred of those in charge. He took great delight in seeing their fear when they fell from power. In direct contrast to the idea that he acts in the best interest of humanity, he told Erwin that he does not care if the revolution helps humanity or not.
Zachary shows a demented, psychopathic side to his personality when he subjects an official to inhumane torture, laughing while forcing him to drink his own urine.
Zachary first appears when he arrives at the court place when Eren Jaeger is put on trial. At the courtroom, he confirms who Eren is and then announces that the sole right of judgment rests with himself. Eren will either survive and go with the Scout Regiment, or he will go the Military Police Regiment and be killed.
Zachary calms Pastor Nick down after he called for Eren's immediate destruction and then offers Erwin the chance to speak. The latter expresses his plan to use Eren's powers to retake the Wall Maria. A merchant and Levi then argue about the sealing of the gates. Nick interrupts again and begins arguing with the merchant, prompting another call for order from Zachary.
Zachary asks if Eren can use his powers for good, Eren replies yes and Zachary counters that Eren reportedly attacked Mikasa Ackermann, another soldier. She reluctantly admits that he did attack her but also requests that it be taken into consideration that Eren saved her life twice before that.
Nile disagrees with this by citing an incident from Eren's past where he stabbed two robbers. Eren begins to get angry and is pointed guns at by the Military Police but Levi arrives and begins to beat him. Erwin then offers a proposition to Zachary: the Scout Regiment will take Eren and assess his usefulness to humanity and if anything goes wrong, Levi will kill him. Zachary agrees with this deal and announces that Eren will join the Scout Regiment and return to trial after his ability has been assessed.
Zachary takes part in Erwin Smith and Dot Pyxis's plan to overthrow the government. After the government officials, believing that the Walls have been breached, demand that no refugees be allowed into Sina, Zachary barges in and reveals that the breach of Wall Rose was a ruse. Erwin is released and the officials are then taken into custody for valuing their wealth over the lives of the people.
After announcing to the people that the old regime has fallen and the royal capitals administrative district have been conquered, Zachary and Erwin board a carriage. Erwin voices his doubts that their decision to overthrow the government was best for humanity, and Zachary comments that with the current state of things, death may have been a preferable option.
He goes on to reveal that he himself has long since despised the government, and has waited his entire life for this day. He appears rather amused as he recalls the hysteria that the arrested officials displayed, remarking that he has been contriving various methods of humiliating them. He claims that he does not care whether or not the revolution benefits humanity, acknowledging that this puts his morality into question, though he believes Erwin is no different from him.
Soon after, he is seen torturing Aurille by hanging him upside down and forcing him to drink his own urine. As Zachary describes the different ways he plans to torture the official, he praises the torture device as the greatest work of art ever conceived. The official taunts him, saying that he has slave's blood and the official's royal blood will enable him to erase Zachary's memories to the point where he could not remember how to defecate, but Zachary silences him by plugging the device's tube into the official's mouth as he lets out a burst of laughter.
After the attack on Orvud District, Zachary and the commanders of the various military branches crown the true heir to the throne, Historia Reiss, as the new queen of the Walls. The officials then kneel in front of Historia, and she bows to them before turning to the crowd and offering her heart in a salute as they cheer.
Zachary oversees a meeting with the military's other commanders to decide what to do with the Titan injection that Kenny Ackermann gave to Levi. With no way to properly analyze the contents of the bottle, the brass decides to entrust it to Levi.
As Armin Arlelt explains his plan for victory against the Colossal Titan to Eren, Zachary sits with the other military leaders drinking tea and worrying if the operation to retake Wall Maria and humanity's land will be successful.
As the military leaders wait for the Scouts' return, Nile tells them about a theory of Erwin's that he had laughed at when they were youths, and Zachary encourages him to apologize to Erwin once he has returned. Shortly afterward, they are informed by Anka that the Scouts have returned.
After the Scouts' return, Zachary is one of three military commanders, including Historia, to preside over a meeting to debrief them on the military's plans moving forward.
As commander-in-chief, he is the highest authority within the military structure. The three military divisions, the Garrison, the Military Police, and the Scout Regiment are all officially under his command, despite having their own leaders. In matters of military law, he is the sole decision maker that cannot be argued against. Within the context of a Military Court, he has the power to condemn the defendant to death if he chooses.