Dirk was a member of the Survey Corps and the team leader of Squad Dirk during the Corps' mission to reclaim Wall Maria.
Dirk was a man of average height and build, with long, light-colored hair that he kept pushed back out of his eyes. He also had a thin beard on his chin and cheekbones, and a small mustache. When not wearing the standard Survey Corps uniform his attire consisted of a white button-up shirt, dark pants, and a black vest.
Dirk appeared to be a rather calm, serious man. When violence erupted among the other soldiers, Dirk only made a casual note of their behavior, and made no effort to break up the fighting or intervene in any way. He seemed to be a trusted subordinate, as he personally escorted and advised Erwin Smith during the Survey Corps' mission to Shiganshina.
Appearance: Dirk was a man of average height and build, with long, light-colored hair that he kept pushed back out of his eyes. He also had a thin beard on his chin and cheekbones, and a small mustache.
Personality: Dirk appeared to be a rather calm, serious man. When violence erupted among the other soldiers, Dirk only made a casual note of their behavior, and made no effort to break up the fighting or intervene in any way.
Dirk in the story: After the Survey Corps is framed for the murder of Dimo Reeves and his subordinates, Dirk is among the Survey Corps members imprisoned while Erwin is put on trial. Dirk attends a meeting between Erwin and the other Survey Corps officers to discuss the recent revelations involving Eren Yeager's father Grisha. After the meeting, Dirk suggests possibly punishing Keith Shadis for withholding information from the Military, but Hange Zoë dismisses the notion, claiming that Shadis is not worth the effort.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.