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The Marley Public Security is responsible for maintaining order within the Eldian internment zones such as Liberio on the continental mainland. It includes restricting movement in and out of the ghettos, countering Eldian dissident movements, and sending undesirable Eldians to Paradis Island where they are turned into Pure Titans. Its soldiers are armed so they can assist in fighting within the internment zones.
In the year 830, the PS were tasked with recruiting Warriors from among the Eldian children aged 5 - 7 in service of the Marley government with the promise of designating the families of the chosen children as honorary Marleyans. Zeke Jaeger, Bertholdt Hoover, Reiner Braun, Annie Leonhart, and Marcel Galliard ended up selected among those children.
Despite its power, the PS had been actually heavily infiltrated by the Eldian Restoration movement to the extent that the latter was able to get its hands on censored historical records and important government documents. For example, Eren Kruger, aka "the Owl," was a high-value Eldian agent who managed to become at least on par with Sergeant Major Gross within the PS command hierarchy. It was not until Zeke Jaeger's defection that the PS was able to weed out most of the conspirators.
PS personnel is only seen in and around the internment zone, suggesting that it is their sole area of jurisdiction. However, it does share the use of a wharf located in the coast of Paradis Island with the military, with the PS using it for the purpose of exiling undesirable Eldian prisoners into Paradis Island and the military uses it to dock heavy transport ships.
The PS turns a blind eye to the brutal behaviors against the Eldians committed by some of its soldiers. For example, PS member Gross had his sons sadistically kill Fay Jaeger by unleashing a pack of dogs upon her for entertainment. The death did not seem to hurt his career, as by the time of Grisha Jaeger's exile to Paradis Island he had attained, or still kept, the rank of Sergeant Major. To crush the Restorationists, the PS also did not shy from using tortures to obtain confessions from captured dissenters. When he was being interrogated, Grisha had all his fingers cut off by the PS.
Others, however, show more decent attitude, or even genuine care, for the people under their watch, with whom they became familiar with due to day-to-day interactions. For example, in the year 854, there were a pair of gate guards who were close enough with the Warrior candidates that they often made small talks during the latter's daily passages from the gate. They were even aware of Falco Grice's affection toward Gabi Braun. During the attack on Liberio, the duo tried to stop Gabi from running into the fighting at a street checkpoint, urged her to go home for safety instead. When they suddenly came under attack by Scout Regiment infiltrators, one of the guards pushed her away from incoming fires before both men were shot dead by Sasha Braus.
The regular PS uniform consists of a soft edge peaked cap with a leather visor, navy blue trench coat, a "Sam Browne" style leather belt, a white shirt which is worn with a black tie, and a pair of leather jackboots. The high-ranking officers however, wear a gray trench coat. Its troopers are armed with modern small-arm weaponry such as bolt-action rifles and revolvers.
The Marley security force in Kruger and Ymir's time wore a "Napoleonic" style uniform which differs to the modern one by having a kepi, a tunic with two rows of buttons, leather shoulder belts, a waist belt which carries ammo pouches, and a pair of epaulets for higher ranking personnel. The infantry weapons of the time seem to be cap-lock muskets.
The PS operated at least one steamboat for transportation to the coast of Paradis Island where an outpost is located, though the boat was later destroyed by Eren Kruger. The outpost itself is built with a pier by the foot of a 30-meter wall surrounding a small segment of the coastline.
At some point between year 743 and 817, the PS, or one of its predecessor organizations, crushed a revolutionary army formed by the Eldian royal family who had stayed on the Marleyan mainland. Eren Kruger's family was burned to death by Marleyan soldiers before his eyes in that event, which made him vow to restore Eldia into its former glory. When Kruger reached adulthood, he managed to join the PS by faking the prerequisite blood test with the help of a doctor sympathetic to the Eldian cause. Later, he organized a new resistance group and supplied it with sensitive information from inside the PS.
At around the same period, the Marleyan security force uncovered an Eldian cult which had Ymir as its figure of worship. She and her fellow cultists were exiled to Paradis Island where they were converted into Titans.
In 817, PS Sergeant Major Gross' sons murdered Fay Jaeger, Grisha's sister. The event traumatized Grisha and influenced him to later become the leading figure within the Eldia Restoration Movement.
In 832, the PS began recruiting Eldian children to be trained as Warriors. In order to gain control of one of the Titans, Grisha sent Zeke to become one of the recruits, which he succeeded. However, Zeke betrayed the identities of his parents to the Marleyans, allowing the PS to capture most of the Restorationists.
PS officers shipped the prisoners to a borderline outpost in Paradis Island to be turned into Titans. During the process, Kruger revealed his true identity to Grisha and transformed into the Attack Titan. The ensuing skirmish led to the loss of the entire PS escort detail and a steamboat.
The PS assists the Marleyan military both in evacuating casualties and preventing civilian access into combat zones during the Raid on Liberio. The lightly-armed soldiers fare poorly against the Scout Regiment attackers. At least two troopers were killed in an ambush.