Leviathan

Leviathan (レヴィアタン) is advanced-level furniture created by Beatrice. Leviathan often speaks to Lucifer on behalf of the younger sisters. She is particularly attached to Kyrie Ushiromiya due to Kyrie's long-standing jealousy of Asumu Ushiromiya.

Background
When the Seven Stakes of Purgatory were growing in their youth, Leviathan was the slowest to grow. Her envy of her sisters lasted seven days and seven nights, ultimately culminating in her envy giving her power eclipsing the rest of the stakes.

Banquet of the Golden Witch
In, Leviathan is called upon to kill Kyrie Ushiromiya. She uses her envy of her sisters as power, however Kyrie is able to use her envy of Asumu Ushiromiya to gain speed far greater than Leviathan. Ultimately, Kyrie manages to shoot and kill Leviathan though she is later killed by the Chiester Sisters.

Dawn of the Golden Witch
Leviathan appears as Kyrie Ushiromiya's ally in and assists her in a battle with Jessica Ushiromiya.

Trivia

 * Leviathan is based off of the biblical character Leviathan and the corresponding sin of Envy.
 * There is another being named "Leviathan" that appears in as a familiar of Bernkastel.