Mayu Otsuka is a supporting character in the anime and original video animation "Darker than Black." She works as a liaison officer between Section 4 of the Public Security Bureau and the National Astronomical Observatory, also referred to as the Astronomy Bureau. Her role involves coordinating information and cooperation between the two organizations during investigations into Contractor-related incidents, such as the death of the Contractor named Louis.

In her personal life, Otsuka is an otaku who enjoys anime and manga. She writes fanfiction based on anime and manga series, was a member of the manga club during her university years, and decorates her room with anime posters, figurines, and trophies. At one point, she is particularly invested in an anime called "Rose of Maurice."

Otsuka appears throughout the first season of the television series as a minor but recurring figure, often attending briefings alongside officials like Yoshimitsu Horai and Misaki Kirihara. She also plays a more prominent role in the original video animation "Darker than Black: Beneath the Fully Bloomed Cherry Blossoms," where she is a main character. She does not possess any supernatural abilities; she is a normal human fulfilling her duties as a police officer and liaison. Her professional demeanor and personal hobby of anime fandom define her character, but she undergoes no significant personal transformation or deep character arc over the course of the story.