Keigo Hayama is a supporting character in the anime, serving as the older step-brother and guardian of the protagonist, Nayu Hayama. He is a young adult with brown hair that is cut to chin-length and blue eyes, and is frequently depicted wearing glasses along with casual household attire such as an apron and slippers.

His background is central to understanding his role and relationship with Nayu. Keigo is not a blood relative but became her step-brother following the death of his father and stepmother, who was Nayu's mother, in an accident. After this tragedy, Keigo took on the responsibility of caring for Nayu, and they moved in with their grandmother. Their grandmother was a notable underwear designer, and it is from her that Nayu inherited her passion for and knowledge of lingerie. In the present day, Keigo works as a professional underwear designer, a profession directly influenced by his family history.

In terms of personality, Keigo is portrayed as a calm, patient, and deeply caring guardian. He is very loving toward his younger sister and is attuned to her emotional state. In one instance, when Nayu becomes withdrawn and depressed after the school rejects her proposal for an underwear club, Keigo provides quiet, unobtrusive support. He understands that she needs to process her feelings and waits for her to come to him, a moment signaled when she asks him to comb her hair. This interaction highlights his gentle and understanding nature, as he provides comfort without prying.

Keigo plays a crucial role in the story as both Nayu's supporter and a link to the series' thematic foundation. His job as a designer allows him to create prototypes that Nayu tests and gives advice on, making her his informal assistant. His career also creates a major plot development when he receives a job offer that would require the family to move to Kyoto, creating a potential separation between Nayu and her newly formed Underwear Appreciation Society.

His key relationships are primarily with Nayu and Tamaki Mizuno. With Nayu, his bond is one of a devoted guardian who has sacrificed to provide her with a stable home after the loss of their parents. With Tamaki Mizuno, the teacher who becomes the advisor for the Underwear Appreciation Society, Keigo has a prior connection. It is revealed that Mizuno was his junior in high school and harbored a significant crush on him. While Keigo does not initially remember this, their reconnection provides insight into his past and adds a layer of history to the series' adult characters. Mizuno’s lingering feelings for Keigo are a subtle subplot throughout the series.

Keigo does not undergo a dramatic character arc himself, but his situation creates a pivotal moment of development for Nayu. The prospect of his new job and the subsequent move to Kyoto forces Nayu to confront her independence and the strength of her friendships, leading to the series' climax. His journey involves balancing his professional ambitions with his commitment to caring for his sister, ultimately finding a resolution that allows her to continue her education and club activities.

His most notable ability is his expertise as an underwear designer. He possesses professional-level knowledge of garment construction, materials, and fit, which he applies to his work. This practical skill is the engine that drives much of the show's educational component and directly supports Nayu's mission to promote healthy attitudes about underwear.