Heiji Hattori is a 17-year-old high school detective from Osaka, famously known as the Great Detective of the West, a title that positions him as a rival to Shinichi Kudo, the Great Detective of the East. He is the son of Heizo Hattori, the chief of the Osaka Police Department, and his wife Shizuka Hattori. His desire to become a detective was sparked during his childhood by solving simple neighborhood cases, such as finding lost cats, a pursuit in which he reportedly solved over a thousand minor incidents.

In terms of personality, Hattori is characterized by his lighthearted and good-humored nature, often approaching even serious investigations with a confident grin. He is intensely proud of his Osakan heritage, which is reflected in his thick Kansai dialect and his constant enthusiasm for promoting the culture and cuisine of his hometown, especially to his friend Conan Edogawa. He is frequently seen wearing a baseball cap for the local Osaka team, typically with the visor pointed backward, though he turns it forward when he feels particularly challenged. Despite his cheerful demeanor, Hattori can be impulsive, a trait that sometimes clouds his judgment and has led to criticism from fellow detectives who prioritize the preservation of crime scenes. However, he strongly believes that a detective's primary duty is to prevent death, leading him to rush to aid victims before securing evidence.

Hattori originally traveled to Tokyo with the sole purpose of challenging Shinichi Kudo to prove his own superiority. After being soundly defeated in a case where Conan Edogawa temporarily reverted to his true form as Shinichi, he gained immense respect for his rival. During their subsequent encounter, Hattori discovered Conan's secret identity. Since that revelation, he has become Conan's closest ally and best friend, dedicated to protecting his secret and often working together to solve complex cases. This partnership extends to investigating the Black Organization, and Hattori has even impersonated Shinichi on multiple occasions to maintain the illusion of his presence, though his distinctive dialect often makes this difficult. Their relationship is brotherly, with Hattori often teasing Conan by intentionally calling him Kudo in sensitive situations, while also fiercely watching his back.

His most significant personal relationship is with his childhood friend, Kazuha Toyama. Their bond was forged in a memorable childhood incident where Hattori locked them together with a pair of old handcuffs that had no key, forcing them to spend days in close quarters. As a reminder of this event, Kazuha created two matching good-luck charms from a chain link of the cuffs, which both have worn ever since. Though they constantly argue and publicly deny romantic feelings for each other, Hattori is deeply protective of Kazuha and frequently leaves her behind during dangerous investigations out of fear she will be hurt. He also becomes visibly jealous when other boys show interest in her, though he can be oblivious to his own feelings. Despite mocking her belief in superstitions and charms, he places great trust in the charm she gave him, once insisting Conan wear it, which ultimately saved Conan's life from a knife attack while Hattori himself was wounded.

In the film Detective Conan: The Private Eyes Requiem, Hattori is one of several detectives assembled by a mysterious client. Along with Conan Edogawa and fellow high school detective Saguru Hakuba, he is forced to cooperate to solve a series of interconnected cases while those close to him are held hostage as collateral. His father's position as the Osaka Police Chief provides him with unique access to police resources, which he utilizes during the investigation.

Throughout the series, Hattori demonstrates several notable abilities. Beyond his keen deductive skills, he is the captain of his high school kendo team and a tournament contender, occasionally using a wooden sword to subdue criminals. He also possesses an eidetic memory, allowing him to recall complex codes and sequences after only a glance. In addition to his native Japanese, he speaks fluent English. His development as a character is evident in his shift from a boastful rival seeking recognition to a humbled, collaborative partner who prioritizes protecting lives over winning a competition of wits. This moral evolution is cemented by Conan's influence, which taught him that a detective who allows a cornered suspect to commit suicide is no better than a murderer themselves.