Haibara Ai, real name Miyano Shiho, is the inventor of the APTX 4869 poison that shrunk Kudou Shinichi into Edogawa Conan, and a victim of the poison herself. Formerly Sherry of the Black Organization, she is on the run from them and lives with Professor Agasa. She is also one of few characters who knows Shinichi is Conan.
Miyano Shiho and her older sister, Miyano Akemi, were born into the secret crime syndicate "The Black Organization." Her English mother, Miyano Elena, and her Japanese father, Miyano Atsushi, were both scientists working for the Black Organization on an important secret project. While Haibara was young, both her parents died in what was claimed to be an accident. While Akemi lived a normal life, the highly intelligent Haibara was groomed to take over her parents' research. A child prodigy, she was sent to the United States for education at a very young age, and then later came back to Japan to finish her parents' secret project.
Shiho was unhappy that the Black Organization's was using the incomplete APTX 4869 for murder, as that was not the drug's intended purpose. When Shiho received word that her sister Akemi had been killed, but was given no explanation why, she decided to boycott her research until the Black Organization gave her an adequate explanation. Instead, the Organization confined her in a small room to await her execution. With no other way out, she took a pill of APTX 4869 to commit suicide, but instead shrank into an eight year old girl. In her new, smaller body, she escaped from the Black Organization and attempted to reach Shinichi's house, hoping he might be able to help her. Exhausted, she collapsed in front of Shinichi's house, where she was found by Professor Agasa, who took her in. In fact, as it turned out later, Agasa was acquaintances with her parents—especially her father—even though he didn't know at that time that they had children or were part of the Black Organization.
To prevent the knowledge of her transfiguration from being discovered, Shiho and Professor Agasa create her alias Haibara Ai. As Haibara, she attends Teitan Elementary School with Kudou Shinichi's shrunken form, Edogawa Conan. Following her first adventure with the Detective Boys in which they investigate the disappearance of a schoolmate's brother, she reveals her true identity to Conan and the fact that she had developed the APTX 4869. At first stunned, then outraged, Conan protests her presence, but as time passes, he accepts Haibara as a friend and fellow victim, though her cynicism and secretive air still rail him at times. Haibara also joins the ranks of the Detective Boys, and although she does not think too highly of her teammates' immature antics, she has come to value them all as close friends.
Haibara has a calm and cool personality, sarcastic wit, and high intelligence. When freshly escaped from the Black Organization, she was cynical and pushed others away. Haibara has since defrosted by embracing her new child life and has established strong relationships with Conan, the Detective Boys, Agasa, and Ran.
She tries to keep Conan from doing reckless things that may give away their identity, though often she is surprised by his actions which usually result in gaining the information he needs while also keeping their identities secret. However, whenever Conan is reckless in regards to keeping his identity from Ran, Haibara is the one who plans ahead in order to keep him from being revealed.