The Military Police Brigade (憲兵団 Kenpeidan?) is the most prestigious due to their work allowing them to live safely within the Inner Wall, abundant resources, and higher authority over other military sectors. However, in reality it is the most corrupted military sector due to a lack of oversight compared to the other sectors of the military. Due to their misuse and abuse of authority, the Military Police Brigade is also at odds with Garrison and especially with the Survey Corps. Only the top ten alumni from trainee squads can apply for the Military Police Brigade. However, according to the conversation between Hitch Dreyse and Boris Feulner, there are other (illicit) ways to get in the sector such as fraternization, favoritism, etc.
Duty
The Military Police Brigade is the primary police force within human territory. Not only do they maintain order, they serve as the king's guard. Since they do not normally come in contact with Titans, they must have special permission to use 3D Maneuver Gear and usually carry firearms such as rifles instead. They are also in charge of tax collection and land distribution as seen in Ch. 15 when a couple of policemen monitored farm production. They usually abuse these duties to exploit resources and food from civilians.
In Episode 25, Wall, their assigned duty was changed due to the mass destruction caused by Eren Yeager and Annie Leonhart. From guarding the Survey Corps, they were ordered by Nile Dok to evacuate all civilians and bringing Commander Erwin Smith on the trial to set punishment for his plan to capture Annie.
However these duties were abused by the Military Police Brigade officials, as it is in their nature.
Structure
The Military Police Brigade is an organization with 2,000 members, with direct command over 3,000 additional personnel from the Garrison. This brings their total potential manpower to 5,000 soldiers. Their forces are divided between the major Districts, with 200 officers assigned per District.
Qualities
Soldiers drink and gamble while on-duty
The Military Police Brigade is intended to be an elite unit made of the most skilled soldiers due to its rigorous standards. The reality however is a lazy, corrupt system that selfishly benefits from the powers granted to it, with these powers being abused and taken unnecessarily advantage of. Though they are from the top ten graduates from the trainee squads, they are inexperienced as they only stay in the safety of Wall Sina and never directly engage in battle with Titans.
The senior officers have been seen spending all their time gambling and drinking while leaving recruits to do all the work. They also pilfer taxes and misappropriate land as a recruit, Marlo Sand mentions. This corruption is well-known with many people (even to Wall Sina citizens) criticizing their lack of care for anyone other than themselves. Refugees and forced laborers (called producers) hold simmering hostility against them.
However, many trainees still dream of joining the Military Police Brigade because it will allow them to live within Wall Sina and safety. Also, being so close to the center of human territory, most policemen have relatively little experience fighting Titans as compared to the Survey Corps or the Garrison. This, together with corruption, loose discipline, contempt from public, abuse of power makes this military branch the most incompetent out of the three.
However, it is later revealed that the King becomes tougher on the Military Police for their lack of diligence in dealing with their corruption, lack of discipline, pilfering of taxes and mismanagement of lands. He orders them to leave the safety of Wall Sina and assist both the Survey Corps and the Garrison to look for the two other traitors in collaboration with Annie. By the end all three factions have banded together to help rescue Eren and Ymir from Reiner Braun and Bertolt Hoover.
During Annie's battle with Eren in the Stohess District, Military Police members were at a loss for what to do, for lack of preparation and overall laziness landed them there. Due to their lateness and ill provision, they could barely handle citizen evacuation and other needs, which ended up bringing more weight on the situation.[1]
First Interior Squad
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Their chain of command is different, and we have no links to them... even we have no idea what they're thinking. There's no one officially in charge of them. No matter what they do, there's no reprisal. | 
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| — Nile Dok, Chief of the Military Police, on the First Interior Squad | ||
The First Interior Squad is introduced in the aftermath of the traitors' being exposed, torturing and murdering Pastor Nick. Their organization is shrouded in mystery, and they actively seek out both Historia Reiss and Eren Yeager for unknown purposes. They have been shown to be far more skilled and disciplined than the rest of the military police, though their main skills are in fighting other humans than Titans. With the Titan Shifters uncovered and driven out, this mysterious group appears to be the new antagonists of the series.
The First Interior Squad operates completely separate from the main group, with their own command structure and duties. Little is known about them, with even Nile Dok admitting to knowing little about them. They are involved in numerous conspiracies that supposedly maintain order within the walls, and stated to be completely above the law. They are rarely seen outside the Capitol. They are later revealed to have a separate division known as the Anti-Human Suppression Squad which use uniquely designed firearms in conjuncture with 3D Maneuver Gear to attack Levi and the Survey Corps as they tried to retrieve Eren and Historia from the Military Police.