Fairy Leviathan is one of the Four Guardians, a group of immensely powerful Reploids derived from X's DNA. She is the commander of the Meikai Gundan (Army of the Dark Oceans) or the Meikai Army for short. Like fellow Guardian Fighting Fefnir, she relishes the feeling of being on the front lines and is rather flirtatious in regards to Zero. Nevertheless, her ties to Neo Arcadia bring her into conflict with the legendary hero throughout the story, serving as a formidable foe whenever they meet.