Leviathan is the Demon Lord of Envy and the second of the Seven Mortal Sins, often affectionately called Levy. She is a demon lord who rules over water and commands aquatic creatures, reflecting her affinity for water-based magic. Leviathan’s background begins in Hell, where she first encounters Lucifer after the fallen angel’s arrival. Instantly infatuated, Leviathan attaches herself to Lucifer as a devoted sidekick, driven by a deep admiration that borders on obsession. This adoration is a defining trait; she craves Lucifer’s attention and becomes intensely jealous when ignored, a jealousy that extends beyond Lucifer to anyone who might draw focus away from her.

Her personality is marked by clinginess and childish behavior, which endears her to some followers but also underscores her immaturity. Despite this, she can be cold and calculating when the situation demands it, and she is fiercely competitive, especially with other demon lords. Her primary motivation is to stay close to Lucifer and earn her affection, which leads her to support Lucifer’s quest to defeat the other ruling demon lords of Hell. After Lucifer’s powers are sealed by the Seven Sins, Leviathan accompanies Lucifer to Earth, where she helps in the campaign to break the curse.

Leviathan’s key relationship is with Lucifer, whom she idolizes, but she also interacts with Maria Totsuka, the human girl who becomes Lucifer’s immortal servant. Through these bonds, Leviathan shows gradual development, becoming more cooperative and slightly less self-centered as she learns to work alongside others. Her notable abilities include manipulation of water, summoning and commanding marine monsters, and summoning a serpent-like creature from within her own body. She also possesses superhuman strength and agility, making her a formidable combatant. Her role in the story is that of a loyal ally and occasional comic relief, whose envy and jealousy add complexity to her character as she struggles with her own flaws while serving her beloved Lucifer.