Darius Zackly was the general of the three military branches: the Survey Corps, the Garrison, and the Military Police Brigade.
Zackly had the typical appearance of an old man. His eyes were sunken and tired-looking, he wore glasses, he had noticeable wrinkles on his face, and his hair was receded and gray. He also had a gray scruffy looking full beard and mustache. In addition, he was rather short which gives him a presence uncommon for a man of his rank. Zackly's casual attire consisted of a plain white shirt with normal dark colored pants. In a more formal setting, he added a bolo tie, much like the three commanders under him, Nile Dok, Erwin Smith, and Dot Pixis. Over this, he wore a brown trench coat that had a black shield emblem with a white border and cross.
When he was first introduced, Zackly 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. Similarly, he overthrew the monarchy with an incredibly casual demeanor. This indicates that Zackly had no time for drama and was solely focused on carrying out his judgments. He remained ever serious but at the same time appeared tired as if his age was catching up to him.
Appearance: Zackly had the typical appearance of an old man. His eyes were sunken and tired-looking, he wore glasses, he had noticeable wrinkles on his face, and his hair was receded and gray.
Personality: When he was first introduced, Zackly 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.
Darius Zackly in the story: Zackly first appears when Eren Yeager is put on trial. 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 Survey Corps, or he will go the Military Police Brigade and be killed.
Abilities: As commander-in-chief, he was the highest authority within the military structure. The three military divisions, the Garrison, the Military Police, and the Survey Corps were all officially under his command, despite having their own leaders.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.