Ayumi Yoshida is a young student at Teitan Elementary School, where she is a classmate of Conan Edogawa, Genta Kojima, and Mitsuhiko Tsuburaya. She is approximately six to seven years old and lives in a large apartment with her mother. Ayumi first befriends Conan when he joins her class, and she is instrumental in convincing him to join her and her friends in investigating a supposedly haunted mansion. The successful resolution of that case, followed by an adventure hunting for stolen gold where Conan saves the group, leads Ayumi, Genta, and Mitsuhiko to officially form the nucleus of an amateur detective group known as the Detective Boys, with Ayumi being the sole female member until Ai Haibara joins their ranks later on.

In terms of personality, Ayumi is portrayed as a primarily naive and innocent little girl who possesses a strong sense of adventure and acts as the cheerful spirit of her group. She demonstrates growing courage as the series progresses and is known for being kind, compassionate, and socially proactive, often being the first to approach and befriend new classmates like Ai Haibara. Her curiosity is a driving force for many of the Detective Boys' investigations, as she is always eager to explore mysteries. Ayumi also has a sensitive side, particularly regarding the emotions and relationships of those around her, and she shows a surprising amount of insight during cases, occasionally making simple yet crucial observations that help move an investigation forward. For example, her ability to notice small, unusual details in a person's appearance or to think creatively about a code has proven valuable on several occasions.

Ayumi's primary motivation is her desire for adventure and to be a helpful member of the Detective Boys, often hoping to solve mysteries alongside her friends. A significant part of her personal drive is her deep affection for Conan Edogawa. She has a very open and serious crush on him, even dreaming of marrying him one day, which Conan himself finds rather uncomfortable. While she is oblivious to the fact that both Genta and Mitsuhiko also have crushes on her, she often notices Conan's interactions with Ai Haibara and has privately wondered if he might have feelings for her instead, given how often he tries to protect Ai and speaks with her in private. Despite any initial jealousy, Ayumi works hard to become close friends with Haibara, and her persistent kindness eventually pays off when Haibara allows Ayumi the special privilege of calling her by the more informal and affectionate name "Ai-chan". This friendship deepens over time, with Haibara coming to see Ayumi as a little sister figure.

Ayumi's role in the story is primarily as a core member of the Detective Boys, placing her at the center of many cases that the group investigates together. Through these adventures, her character shows subtle development. While she begins as a very naive child, her repeated exposure to dangerous criminal situations, from which she is often rescued by Conan, gradually builds her resilience and courage. She learns to be more observant and contributes more actively to solving mysteries, moving from simply being the heart of the team to occasionally providing the key insight that leads to a breakthrough. Her emotional growth is also evident in her maturing friendship with Ai Haibara, moving from a one-sided effort to befriend a distant classmate to a genuine, mutual bond of trust and care. Ayumi's notable abilities include her sharp eye for detail and her talent for making simple but accurate observations that others might overlook. She is also known to be a skilled piano player and has a deep fondness for popular culture icons like the superhero Kamen Rider and the monster Gomera. Her open and caring nature is her greatest strength, allowing her to connect with others and inspire loyalty among her friends.