Azuha Hoshino is a 19-year-old university student who serves as one of the primary supporting characters in the series. She resides on the fourth floor of the same building that houses the Nagumo Detective Agency, a space she has transformed into a chaotic yet functional laboratory. Azuha is a self-proclaimed science agent and the self-anointed invention king of Shinjuku, titles she proudly uses to introduce herself to others.

In terms of appearance, Azuha is a young woman with a striking and memorable look. She has long, vivid red hair that reaches her waist, which she almost always keeps tied up in a high ponytail. Her most distinctive feature is her eyes, which have unusual spiral-patterned irises, a trait inherited from her father that immediately signals her eccentric nature. She possesses a noticeable figure, which occasionally becomes the subject of comedic remarks within the story. Her typical outfit consists of a dark dress shirt and matching skirt, often paired with a tie and an ever-present white lab coat, solidifying her visual identity as an inventor.

Azuha's personality is defined by a potent combination of genius-level creativity, profound eccentricity, and a laid-back, almost indolent demeanor. Her worldview is entirely unique, leading her to describe common things in bizarre, literal terms. She often refers to the high school protagonist Mashiro Nakanishi as a fresh high school girl and calls beef dead cow meat. Her speech is peppered with such unconventional phrasing, making her interactions with others consistently unpredictable. Despite her high intellect, she is extremely lazy and disorganized. Her room is famously cluttered with garbage bags filled mostly with paper, creating what she considers a controlled scientific mess, though she insists it is not dirty. She prefers to have groceries and supplies delivered directly to her door to avoid errands. However, beneath this lazy exterior, she is a practical and direct person who, despite her odd logic, is not afraid to speak her mind or show genuine surprise and exasperation, especially in response to Mashiro's actions.

Azuha's primary motivation is her passion for invention, a talent she inherited from her mysterious mother, Kaguya Hoshino. She dedicates most of her time to creating a steady stream of revolutionary gadgets and devices. Her inventions are always brilliantly designed to serve a specific, useful purpose for detective work, but they are just as reliably plagued by catastrophic or ridiculous side effects. For example, she has created adhesive bandages that force the wearer to loudly declare their preferred body part on the opposite sex, vitamins that transform anyone under the age of fifty into a cold-blooded animal, and shoes that explode when used to kick something. These flawed creations are a central source of both conflict and comedy in the narrative. Her deep-seated motivation is tied to her family legacy; her mother is an even more enigmatic and eccentric inventor, and the truth about her father, who is said to have disappeared or died shortly after her birth, remains a mystery connected to the larger story.

Azuha's role in the story is that of a crucial, if unpredictable, ally to the main detective team. She debuts in the fourteenth episode during a case titled Mashiro and the Star Agent, and quickly becomes a recurring fixture. The protagonists frequently turn to her for scientific support, and while her gadgets always complicate the situation, they often provide the chaotic key to resolving the case. Her role is less about direct investigation and more about providing the tools, however faulty, that drive the plot forward in unexpected directions.

Her key relationships are centered around the main cast. She shares a close friendship and working partnership with Mashiro Nakanishi. Mashiro often seeks out her inventions for help with investigations, and the two also spend personal time together, such as going out to eat grilled meat, which Azuha calls the consumption of dead cow. She has a long-standing but initially distant connection to Keiichiro Nagumo, who is an old friend of her mother. After a long period of no contact, they reconnect during the story, and their interactions often involve her providing medical supplies or treating his injuries. She becomes friends with Hana Kazamaki after a girls' night out with Mashiro; while Hana is initially wary of Azuha's strangeness, they eventually develop a relaxed and amicable relationship. The most significant but less visible relationships are with her family: her mother Kaguya, whose talents she inherited and who is tied to the series central mystery known as the Hamelin incident, and her late father, from whom she gets her red hair and spiral eyes.

Throughout the series, Azuha shows minimal traditional character development, as her core traits remain consistent. However, her backstory and family secrets are gradually revealed over the course of the long-running serialization, adding depth to her motivations and explaining the source of her unusual talents. Her growth is less about changing who she is and more about the audience and other characters uncovering the mysteries of her past.

Azuha's notable abilities lie almost entirely in her inventive genius. She has a remarkable talent for engineering and science, allowing her to build incredibly sophisticated devices. Her adaptability is also notable; she remains calm and unflappable in the face of most situations, reacting to the disastrous results of her own inventions with either brilliant improvisation or ironic amusement rather than panic. Her unconventional mind allows her to approach problems from angles no one else would consider, making her an invaluable, if dangerous, asset.