Who is Fairy Leviathan?
Fairy Leviathan is a character that appeared in three of the four Mega Man Zero games. After the original X sealed the Dark Elf inside him and disappeared into status, a human scientist named Ciel used X's DNA to not only just create a clone of X, but also four guardians which have schematics based off of X's abilities.
Fairy Leviathan was one of those four guardians, and her schematics were based off of X's underwater combat abilities. Sometime after, she became a military general in Neo Arcadia and commanded the Deep Sea Squadron (Meikai Gundan in Japan), which served as Neo Arcadia's version of the U.S. Navy.