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 not particularly fond of violence, but she seems to enjoy battling against Zero, being playful with him. The designers of the Mega Man Zero series have expressly stated that they wish they could have worked on both Fefnir and Leviathan more to "flesh out" their personalities. This leads some to believe they may have much deeper personalities than the games alone display. As a result, Leviathan may act flirty at times, but rarely does so enough to fully "flesh out" her personality; unlike Fefnir's, which can be obviously seen immediately from his "violent tendencies". She prefers to stay underwater, as she says "land is too dusty" for her.
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.
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 by her forces.
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.
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), and although the spikes are removed, she is still weak to Flame. 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 the Baby Elves Elpizo had with him, along with the Dark Elf, which increased her urge to fight Zero and awakened this form. Zero still manages to defeat her, and she leaves reluctantly.
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.