Ruri-chan

Ruri-chan is the main protagonist of an anime called "The Magical Ruri Hana: Demon Girl", which is one of Leviathan's favorite anime.

She's a demon who moves to the human world in order to learn more about it. However, she also loses some of her magical powers and turns into a young girl and only regains her original form when she returns to Devildom.

Trivia

 * Ruri-chan has a companion called Azuki-tan, who's an imagawayaki (pancake filled with red bean paste).
 * In a facebook post, it's said that Leviathan admires how she's given up her true form just to learn more about humans and that he'd support her regardless of which form she's in.
 * It also mentions how Asmodeus prefers Ruri-chan in her true, grown-up form.