Undine operates under contract at Akutabe's detective agency as a secretary. Classified as a demon embodying Jealousy, this nature dictates her human-world interactions. Her powers manifest when jealousy surfaces—often provoked by attention toward Akutabe—cursing individuals, typically women, into unrecognizable forms. These curses dissolve upon her experiencing happiness.

Physically, she shifts between two states. Her diminutive form exhibits pale green skin, medium-length blonde hair, and a green fish tail, appearing more humanoid than fellow demons. In her hell form, she retains mermaid traits while presenting heightened beauty: longer blonde hair frames eyes often veiled by her fringe.

Her personality revolves around an obsessive, unreciprocated fixation on Akutabe. She relentlessly pursues his affection despite his rejections and exploitation of her feelings for agency tasks. This fuels volatile jealousy toward women near Akutabe. Contrastingly, in Hell, she displays devoted care as a daughter to her ailing mother.

Originally bound to summoner Himoi before transferring to Akutabe, Undine maintains the ability to assume human guise despite her demonic essence.