Kaiko Mikuniyama is a supporting character in the story, recognized as a highly talented manga artist hired to adapt the light novel series written by the protagonist, Itsuki Hashima. She is a young woman with green eyes and black hair that falls to her chest. Her professional skill is considerable; her artistic style is noted for matching the tone of Itsuki's original work exceptionally well, leading them to have a positive and productive working relationship upon their first meeting.

Beneath her refined and composed exterior lies an extreme and unabashed personal obsession. Kaiko shares Itsuki's passion for little sister characters, but her most defining trait is an intense fetish for underwear, specifically panties. This fascination is not merely an interest but a core part of her daily life and creative process. She wears a bow on her head that is actually made from a pair of panties, and while working on her manga illustrations, she removes this bow and places the garment over her face.

Kaiko's background explains the root of her unusual fixation. Her family runs an underwear store, which is the primary influence for her deep-seated appreciation for the item. This background also creates a source of personal conflict, as her father holds a very conservative, traditional worldview. He strongly disapproves of manga as a medium, viewing it as insignificant and his daughter's chosen career as shameful. Over the course of the story, he eventually comes to accept her profession.

In the narrative, Kaiko serves as a collaborator and a friendly rival. She is initially in competition with another artist, Puriketsu, for the contract to draw the manga version of Itsuki's series. Despite this competition, she integrates into the main circle of characters, maintaining a friendly but slightly more distant relationship with them compared to the core group, as she is not always present for their casual gatherings. Her compatibility with Itsuki is frequently highlighted, suggesting a strong alternative partnership that the story occasionally explores, including in a parallel world scenario where she takes on a more central heroine role.

Her development spans both her career and personal life. Professionally, her abilities grow to the point where she creates a masterpiece manga adaptation that is considered superior to the original light novel it was based on. This success leads to international recognition, including an invitation to Taiwan and a fan meeting where her unique persona earns her the nickname Panty Manga Teacher. Later in her career, after the main events of the series, her personal life takes a significant turn when she enters a romantic relationship with a female assistant, marking a notable change in her character. Her notable abilities, beyond her artistic talent, include her creative process, which is intrinsically linked to her peculiar habit, and her deep understanding of the little sister and underwear motifs that she weaves into her work.