Levia-san is a character from the anime Muromi-san, whose true identity is the Leviathan, a giant sea serpent recorded in the Old Testament. Despite her fearsome origins, she appears as a mermaid with very long blue hair and purple eyes, and she speaks with the Kokura dialect, reflecting her background from Kitakyushu. Initially a fierce monster, she was reformed by Muromi-san and has since become much more friendly and approachable.

In terms of personality, Levia-san embodies a reliable and free-spirited big sister figure. She is more mature and level-headed than Muromi, often mentoring both Muromi and another mermaid named Hii-chan with seasoned wisdom. However, she still retains a demonically wild side that can emerge in critical moments, a remnant of her past as a divine enforcer who once unleashed cataclysms without hesitation. This duality makes her both a grounding presence and a reminder of the immense power she holds.

Her role in the story is primarily as a supporting character and friend to the main cast. She frequently appears at the dock where the group gathers, offering advice, sharing her knowledge, or participating in their comedic adventures. Her motivations are not deeply explored, but she seems driven by a desire to enjoy her current peaceful life, guide her younger friends, and occasionally indulge in her older, more chaotic instincts.

Key relationships include her close bond with Muromi, whom she treats as a junior and often helps out of trouble. She also interacts with Takurou Mukoujima, the human protagonist, and other mythological creatures like Yeti and Harpy. Her development is mainly a shift from a feared monster to a beloved companion, though she never fully loses her dangerous edge. Notable abilities stem from her nature as the Leviathan, making her one of the most powerful beings in the series. She possesses immense physical strength and the latent capability to cause widespread destruction, but she rarely displays these powers in everyday situations.