Four Guardians

Left to right: Harpuia, Leviathan, Fefnir, and Phantom.

The Four Guardians, known as the Four Heavenly Kings (四天王) in Japan, is a group appearing in the Zero era. They are four powerful Reploids that were created from the DNA of X to help heal the planet's environment after the Elf Wars. They also serve in military capacities, leading the armies of Neo Arcadia. In the ZX era, all four of them return as Biometal.

In the Games

Zero era

Mega Man Zero

The Four Guardians first appeared in Mega Man Zero, where they serve as four of the game's bosses. They are:

• Sage Harpuia, leader of the Strong Air Battalion.
• Fairy Leviathan, leader of the Deep Sea Squadron.
• Fighting Fefnir, leader of the Scorched Earth Squadron
• Hidden Phantom, leader of the Cutting Shadow Squadron.

Once Zero begins turning the tide of the Resistance's conflict with Neo Arcadia, the Four Guardians target him directly. Zero fights and defeats them all in different Missions, forcing them to retreat.

In the Neo Arcadia Core stage, the Four Guardians return as part of the boss attack. They again retreat upon being defeated, save for Phantom, who self-destructs in hopes of taking Zero with him, though this fails. Harpuia, Leviathan, and Fefnir then appear before Copy X to defend him from Zero, but he orders them to stand down so he can fight Zero alone.

Mega Man Zero 2

The Guardians (now reduced to three) play a larger role in Mega Man Zero 2. With Copy X destroyed, Harpuia is forced to take his place and act as an interim leader for Neo Arcadia. The three continue to lead the Neo Arcadian military, though Phantom's Cutting Shadow Squadron is disbanded and its members are integrated into the other three battalions.

Elpizo's Operation Righteous Strike is brought to a halt by the Guardians, who destroy the invading force and corner Elpizo. Later, while pursuing the renegade Elpizo, Zero fights the Guardians across various missions again. Once Elpizo infiltrates the temples of Neo Arcadia, Leviathan and Fefnir both use their Armed Phenomenon forms to fight Zero. Harpuia also uses his Armed Phenomenon, but only after Elpizo mind controls him using the Baby Elves. Though defeated, all three Guardians survive.

Mega Man Zero 3

The Guardians have a smaller role in Mega Man Zero 3. The three investigate the Derelict Spacecraft only for Leviathan and Fefnir to be injured by Omega. Once Zero and Harpuia arrive, the fighting is stopped by the appearance of Dr. Weil and Copy X MK. II, who retake control of Neo Arcadia. The Guardians are demoted and Dr. Weil replaces them as leader of the military. Leviathan and Fefnir stay in recovery, while Harpuia eventually turns on Weil and is granted asylum by the Resistance.

In the Sub Arcadia stage, Zero can enter Cyberspace to find a secret boss battle with Phantom's posthumous data. Upon his defeat, he encourages Zero to go on to destroy Omega. After Zero fights Omega, X and the three Guardians arrive to provide backup. With their help, Zero lands the final blow on Omega, destroying him in a massive explosion. The fate of the Four Guardians after this event is left unknown.

Mega Man Zero 4

The Four Guardians are absent from Mega Man Zero 4. A fan letter asking why was answered by developers in U-Capcom, explaining that the Guardians had disappeared and were working for the sake of the humans elsewhere. That same year, the Mega Man Zero 4 soundtrack booklet contained artwork of the Four Guardians and X watching the fall of Ragnarok, implying they were still alive in some capacity (likely as Cyber-elves, given the presences of X and Phantom).

However, in the book Mega Man Zero Official Complete Works, Takuya Aizu claimed that this image was not canon and that the Guardians died shielding Zero from Omega's explosion. Aizu later backpedaled in the book MM25: Mega Man & Mega Man X Official Complete Works, claiming that Neo Arcadia has no data on them but that they may be doing good for humans simply outside of Neo Arcadia's surveillance.

ZX era

Mega Man ZX

In Mega Man ZX, four Biometal based on the Four Guardians appear. Model H, Model L, Model F, and Model P (plus Model X and Model Z) were all created by Ciel as a countermeasure to the evil Model W, meant to one day empower a righteous Mega Man so they could fight against it. However, these four Biometal falls into the hands of Serpent, who uses them to empower his Pseudoroids instead. Vent/Aile eventually takes them back and uses Model X to Double Megamerge with them, making these four Biometal forms playable.

Mega Man ZX Advent

In Mega Man ZX Advent, Prometheus and Pandora steal the four Biometal from the Guardians and deliver them to four new Mega Men: Aeolus for Model H, Thetis for Model L, Atlas for Model F, and Siarnaq for Model P. These four Mega Men are roped into the Game of Destiny, becoming servants of Albert who seek to empower Model W in order to supposedly become the Mega Man King. Grey/Ashe is able to copy their forms using Model A's A-Trans, again making these four Biometal forms playable. By the end of the game, the four rival Mega Men are defeated, but still retain their respective Biometal and appear to be loyal to Master Thomas instead.