The X Guardians, also known as the Four Guardians, is a group of 4 Reploids that were created from the DNA of the original Mega Man X and they are led by Copy X. They are the secondary antagonists of the first two Mega Man Zero games and the anti-heroes in the third.
The group consists of Sage Harpuia, Fairy Leviathan, Fighting Fefnir, and Hidden Phantom. They are the most elite soldiers, military commanders of Neo Arcadia, and they rule the city in a council alongside the Eight Gentle Judges and Copy X. Each of them wields one of the Ten Shining Weapons and has an Armed Phenomenon form. They were gathered from various areas to counter the Resistence efforts.
The original purpose of the Four Guardians was to restore the lands damaged by the war. To perform their duty, they scattered across the world and acted as the controls of a weather-controlling station called Weather Orbit. Harpuia and Leviathan also partnered to help humans repopulate habitable areas, while Fefnir led Operation Scorched Earth. However, when the Resistence was created, the Four Guardians gathered with a common goal: to protect Neo Arcadia and eliminate the opposition, even though their weapons were not made for that purpose. To that end, they took command of Neo Arcadia's armies: Harpuia led the Strong Air Battalion, Leviathan took over the Deep Sea Squadron and Phantom assumed leadership of the Cutting Shadow Squadron. Fefnir led the Scorched Earth Squadron.
While the Four Guardians initially have the edge over the Resistance, the situation changes with the awakening of Zero, who singlehandely defeats the Four Guardians and stops each attempt from Neo Arcadia to destroy the Resistance. When Zero infiltrates Neo Arcadia itself, the Four Guardians fight the legendary Reploid once again to stop him from reaching Copy X. Despite their hardest efforts, as well as Phantom's sacrifice, they are ultimately unsuccessful in preventing the fall of Copy X at Zero's hands.
Afterwards, Harpuia is elected as Copy X's substitute while covering up his master's demise. One year later, Harpuia unexpectedly saves a broken down Zero by taking him back to the Resistance, but he still joins his fellow guardians to protect Neo Arcadia from Elpizo's Operation Righteous Strike. In retaliation, Harpuia sends a bombardment aircraft to destroy the Resistance, but the attack is stopped by Zero. When Elpizo runs amok with the Baby Elves to release the Dark Elf, the three guardians are eluded by the deranged Reploid and influenced by the Baby Elves to fight Zero, who was in pursuit. Although they are defeated by Zero, Elpizo successfully destroys X's body, releasing the Dark Elf before he is finally stopped.
Two months later, the guardians join forces once again to destroy Omega, who recently returned from exile. However, Fefnir and Leviathan are badly damaged and forced to retreat as Zero joins the fight. Harpuia appears soon afterwards to attack Omega, but the fight is interrupted by Dr. Weil and a revived Copy X, who prioritizes the capture of the Dark Elf. Although Harpuia objects to the return of Dr. Weil and Omega to Neo Arcadia due to their role in the Elf Wars, Copy X justifies that their banishment did not follow official channels. However, the Four Guardians' repeated failures and Harpuia's questioning eventually irritate Copy X, who decides to disband the Four Guardians and transfer their authority to Dr. Weil. When Neo Arcadia's hunt for the Dark Elf results in the destruction of a residential district, an enraged Harpuia attacks Omega, who merges with the Dark Elf and critically damages the ex-guardian. Although Harpuia is saved by Zero and repaired by the Resistance, he decides to continue protecting the humans by himself.
As Zero fights Copy X and Dr. Weil's forces, the Four Guardians are reunited by the original X; after contacting Phantom's soul in Cyberspace, the two approach Fefnir and Leviathan, who remain in critical condition since they were denied repairs after the fight with Omega. Although Phantom stays behind to meet Zero in Sub Arcania, Fefnir and Leviathan move on to join Harpuia despite their condition, and the three eventually help Zero by attacking Omega together in the final battle, and sacrificing their lives to shield Zero from the explosion following Omega's demise.
- The Four Guardians were also called the "Four Heavenly Kings (in Japanese: 四天王, Shitennou), the same name shared by the 4 evil generals of the Negaverse in the Sailor Moon series.
- According to graphic designer Azuma Honda, the Four Guardians were originally supposed to be all blue since they were the replicas of X. However, artist Toru Nakayama gave each of them different colors to emphasize their elemental attributes and help differentiate them in the game. Although the idea was initially met with resistance, it was eventually agreed upon.
- For some time, the fate of the Four Guardians after Mega Man Zero 3 was uncertain, and the only indication was an image in "Vile's Incident" of the original X and the Four Guardians during what seemed to be the fall of Ragnarok. However, it was later revealed in an interview that the guardians had died shielding Zero from Omega's explosion, and that the image in question was just a generous contribution from Nakayama's part. When asked about the timeframe of the illustration, Nakayama expressed that he was unsure, and Takuya Aizu jokingly acknowledged that the timeframe and events of the illustration were a little fuzzy, so they just threw it in there.