Leviathan is a sea creature who appears similarly to an eel. He is largely purple with a turquoise underbelly.
Leviathan's sprite is a modified version of his Final Fantasy V appearance.
Leviathan was one of several espers captured by the Gestahlian Empire during their first raid on the esper world. The captive espers were taken back to the Magitek Research Facility at Vector, and for sixteen years they were experimented on and had their magic drained to develop the Empire's Magitek army.
Short before the Returners infiltrated the facility to shut it down, four espers including Leviathan staged an escape. A year later after the apocalypse in the World of Ruin, Leviathan prowled the waterways between Nikeah and South Figaro, its shadow frightening sailors.
When the party took a ship between Nikeah and South Figaro, Leviathan attacked them and they were pulled under the water. They defeated Leviathan and returned to the ship, and Leviathan's magicite washed onto the decks and was taken by them.
Leviathan is not very useful as an esper, ironically because it teaches a water-elemental spell that is not much weaker than Leviathan himself.
Leviathan teaches only a single spell, but it is very useful for being the only water-elemental spell the party can learn. It is particularly useful against the Gold Dragon, who is weak to water and in its Dragons' Den rematch absorbs magic, but Flood cannot absorbed.
- Flood x2
Leviathan cannot be encountered until the party retrieves the Falcon. After this, taking the ferry between South Figaro and Nikeah will trigger a fight with Leviathan. Upon winning the party receives his magicite.
If Mog is in the party, he can learn the Water Harmony Dance. In versions prior to the Gameboy Advance release the Dance could be permanently lost if not learned in the World of Balance; the Leviathan fight gives players a second chance to acquire it in the World of Ruin.