Leviathan (リバイアサン or レヴィアサン, Ribaiasan or Reviasan)? is a demon in the series.
Leviathan is a sea monster referred to in the Tanakh and the Bible. In demonology, Leviathan is one of the seven princes of Hell and its gatekeeper. The word "leviathan" has become synonymous with any large sea monster or creature. In modern literature (such as the novel Moby Dick) it refers to great whales, and in Modern Hebrew, it means simply "whale." According to Judaic lore, the Behemoth and Leviathan will do battle at the world's end, and kill each other, leaving their bodies to be feasted upon by the chosen peoples who survive.
One of the four new bosses included in the PSP Version, Leviathan can only be fought after beating the game once. Its design is based on the Latter Dragon Type 2 form of Zeed in Devil Summoner: Soul Hackers.
Leviathan appears as the Shadow form of Sae Niijima to be fought as a boss in the Niijima's Palace. The Leviathan is this game however differs from the other variations, in that the Leviathan is a different origin design-wise. In the case for this Shadow, it follows more of the idea of the Leviathan as a political theory made by Thomas Hobbes. A evident reason to this is that the Shadow holds a sword and a gun (which is made to be the sceptre int the original).