Leviathan, is a member of the Four Guardians of Neo Arcadia in the Mega Man Zero series. She was created from Mega Man X's DNA data after the Maverick Wars to clean the world's oceans. Leviathan's schematics are based off X's capabilites of underwater combat. Some time later, she became a military general of Neo Arcadia, commanding the Meikai Army (literally Army of the Dark Oceans), Neo Arcadia's navy force
Like her siblings, her armor is based on that of her biological-data origin, X, with thematic alterations. Leviathan, being the only female, is the most radically different visually, her armor altered to fit her feminine frame. Her armor is colored blue, symbolizing her alignment with water and ice. Her name is derived from that of the biblical sea monster, Leviathan, and typically refers to any large sea creature. Her helmet frames her face in such a manner as to resemble hair, with the sides forming the 'fringe' and a seashell motif forming the 'bangs'. The back is mounted with a long dorsal fin-like jet, which she uses to propel herself through bodies of water. She wields a harpoon-like weapon, the size of which she can alter, and she uses it to create ice and launch torpedoes. She is 5 feet and 4 inches tall, weighing 125 pounds. Her voice is similar to Seychelle Gabriel.
In her Armed Phenomenon form, Leviathan's armor expands to become a submarine in the shape of a gigantic manta ray, becoming much more fitting of the name 'Leviathan'.
Leviathan is flirty and playful and fun. She likes to tease people, especially her brothers, but also people she likes. Giving stupid pet names is a thing for her. Although Leviathan enjoys being laid back and silly, she knows when she needs to get serious, although that doesn’t mean she drops the goofiness off completely. She’s not exactly a fan of violence, and will try to avoid it unless absolutely necessary. She also obviously prefers being in the water to being on land. If you’re really amazing, you might be able to get Leviathan upset, and then she’s gonna take anymore of your bullshit.
As a Reploid able to control the currents and water temperature, Leviathan has paired up with Harpuia before to execute large-scale projects aimed at helping humans re-populate habitable areas.
Meikai Army
At some point, Leviathan, along with her three siblings, became military generals of Neo Arcadia, responsible for the overall safely of Neo Arcadian habitants, both humans and Reploids. Leviathan, given her unrivaled superiority in the water, was appointed to become the leader of the Meikai Army, Neo Arcadia's navy forces. She commanded various water-type forces and Mutos Reploids. The Mutos Reploids Blizzack Staggroff and Poler Kamrous belonged to her forces.
Mega Man Zero
Leviathan first encounters Zero after he thwarts her attempts to hack into the Resistance Base (in the "Stop the Hacking" mission). Despite the fact that the battle with her (both in the mission and near the end in Neo Arcadia's Area X) is underwater, she is weak to Flame (because she is of the Ice Element). She later has a rematch against Zero, but after being defeated, fled the area. She does attempt, alongside Sage Harupia and Fighting Fefnir, to prevent Zero's passage to Copy X, but were forced to stand down under Copy X's orders. Unlike her "brothers", she seemed to be compliant to Copy X's command of leaving him alone with Zero.
Mega Man Zero 2
Zero encounters Leviathan in the same Computer Zone that he fought Poler Kamrous, albeit in a different part of it. As before, the battle is underwater (Leviathan floods the room upon the start of battle), although the spike pits are removed. When going after Elpizo in Yggdrasil, Zero fights against Leviathan in her Armed Phenomenon form within the Temple of Ice, having been given a power boost from the Baby Elves Elpizo had with him, which increased her urge to fight Zero and awakened this form. Zero still manages to defeat her, and she leaves reluctantly.
Mega Man Zero 3
Leviathan took part in a mission to investigate the wreckage of a crashed spaceship that contained the exiled Reploid, Omega at the beginning of Mega Man Zero 3. She and Fefnir were heavily damaged in the battle with Omega. Because of Dr. Weil's return as well as his constant orders to both the maintenance and medical teams, only three people were available to treat her and Fefnir. However, shortly after the transfer, they lost consciousness and were in critical condition, as they later told Harupia. Harpuia later remarked to Copy X that "Fefnir and Leviathan are still undergoing repairs", but neither he nor them were allowed a chance at trying to capture the Dark Elf, due to Copy-X demoting them and giving full command of Neo Arcadia's military to Dr. Weil. In one of the drama tracks from Remastered Tracks Rockman Zero Telos, the original X and Phantom appear to Fefnir and Leviathan and say they must help Zero defeat Omega and Dr. Weil.