The Guards (衛兵, eihei?, translated as Royal Guards by Crunchyroll) are special People of the Earth in the world of Elder Tale who are charged with maintaining security and public order within noncombat zones such as most Adventurer Cities.
Donning special Mobile Armor that grant them abilities beyond those of an adventurer, guards appear whenever an attempt at starting a fight, stealing, or taking someone hostage occurs in a noncombat zone. Offending players will usually be taken to prison for a period of time, and if they attempt to resist, will be killed.
The guards' mobile armor is a large, black suit of armor with long black capes, wielding either a battle-axe or sword. Some Guards are also seen dual-wielding their weapons. They teleport from a ball of blue light and float in mid air whenever players in cities or safe zones are about to fight.
Purpose and Abilities
The guards act as peacekeepers within noncombat zones with where, they will, almost without fail, punish anyone who attempts to start a fight, steal, or keep someone prisoner. If an offending player is detected by the guard system, guards will instantly teleport over to arrest them. While wearing their armor, guards have abilities beyond a level 100 adventurer's, and constantly adjusts to remain more powerful than an adventurer.
Mobile armor grants several different abilities to its wearer. Its helmet has night-vision capabilities, and the armor greatly increases physical strength as well as defense and magical resistance. However, they provide little recovery benefits since they were designed in such a way that the wearer isn't supposed to take damage to begin with. It also seems that one does not need to wear the full suit of armor in order to gain its effects; after Enbart Nelles steals a suit, he only wears part of the helmet, the bracers, and the shin guards. (Notably, however, he's only level 94; it may be that wearing the full suit is necessary for all its stat boosts.)
These abilities are made possible through the technology left behind by the alvs, used together with magical defensive circles inside barrier cities. These magic circles provide the power for the mobile armor to function within the city's boundaries; however, if the armor is taken outside the boundary or if the magic circle is turned off, the armor will be little more than massive, clunky weights dragging the wearer down.
Because of the secrecy regarding the alvs' legacy, all guards are members of the Kunie clan. As the clan also maintains the city's defensive barrier and guard system, this means they also have the ability to cut off the magic supply and deactivate both functions.
After the Catastrophe, several major loopholes are brought up over the course of the series.
Firstly, the guards were designed to check for combat actions, such as using a weapon to attack another Adventurer or Earthling, casting a combat spell against another player, or intentionally inflicting HP damage. As a result, certain actions that only became possible after the Catastrophe—like sexual harassment, enslavement, and other noncombat moves—are not in the guards' detection capabilities. Brigandia and Hamelin were notorious for experimenting with how far they could go before the guards would appear, and using that knowledge to keep their victims in line.
Furthermore, in cases like poisoning or fall damage (for example, dropping a large object onto someone from a great distance), it may not register as an attack depending on the circumstances and intent.
Secondly, the guards' surveillance system can only monitor the actions of Adventurers and People of the Earth; not their fellow guards'. Guards engaging in combat are deemed to be enforcement of security, so even if a guard attacks a person unwarranted, that offense won't be detected. Because of this, Enbart Nelles' actions went undetected by the surveillance system, and he was only discovered when the Kunie did an inventory check and realized his armor was missing.
In episode 2, The Royal Guard was mentioned when Shiroe and Marielle discuss the rampant Player Killing, with a visual of three of Royal Guards appearing out of nowhere to quell the fight between Adventurers.
Fallen Guardian arc
Enbart Nelles becomes possessed by the sword that contained the soul of an Ancient Raid Boss, Lugrius. He kills West Wind Brigade member Kyouko, inciting the rage of Soujiro. For the next week, this "Murderer" kills several Adventurers, including a patrol team from Honesty. In an attempt to defeat it, both Soujiro and later Akatsuki die attacking the Raid Boss-Guard.
Later, a member of the Kunie clan, Kinjo, approaches Raynesia with the news that a Guard had stolen one of the mobile suits. Because of the possessed Guard's ability to use the teleportation system to escape pursuers and find new targets, the Round Table members present are forced to shut off Akiba's Royal Guard System. Not only did this stop the Royal Guards from appearing in case people fought within city walls, it also meant that monsters, which were usually kept from entering the city by the Guards, were now free to enter the city unless stopped by Adventurers themselves.
This article contains plot details for Season 3 and beyond.
Meeting Minami arc
At some point, members of Plant Hwyaden developed a mobile Royal Guard armor—usually impossible to use outside of their designated barrier city. Bestowed on Roreil Dawn, this allowed him to fight on par with and eventually overpower the much more experienced and higher-leveled Mizufa Trude.
Log Horizon: West Wind Brigade
In the Log Horizon: West Wind Brigade spin-off manga, Magus, Passhita, and Coza take advantage of this knowledge to sexually assault Isami and Sara. Because they weren't doing anything that dealt damage, the Royal Guard didn't respond to their actions, and only showed up after Isami attacked Passhita with her sword and drew blood. Soujiro, who appears right as the Guard does, decides to draw the Guard's attention to himself to buy Isami time to escape. Although he sacrifices his own life in exchange, he revives in the Cathedral and the Guards seem to drop her case some time after Souji's death.