Heiji Hattori is a seventeen-year-old high school detective from Osaka who holds the title of the Great Detective of the West, a counterpart to Shinichi Kudo's 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 passion for detective work began in childhood, where he spent his time solving small neighborhood cases, and by his teens he had already cracked over a thousand cases. Beyond his investigative work, he is also the captain of his school's kendo team, a sport in which he is highly skilled.

In appearance, Heiji has dark brown, spiky hair and tanned skin, a trait he claims to have inherited from his paternal grandfather. He is almost always seen wearing a baseball cap for his hometown Osaka team, typically with the visor pointed backward, though he turns it forward when he feels challenged or is deep in thought during an investigation. He speaks with a thick Kansai dialect, which is a source of pride for him as an Osakan citizen but can also be a disadvantage, particularly when he attempts to impersonate someone who speaks standard Japanese.

Heiji's personality is characterized by a lighthearted, good-humored, and often impulsive nature. Unlike the cooler and more collected Shinichi Kudo, Heiji is hot-blooded and quick to act, which sometimes clouds his judgment. This impulsiveness was directly addressed in the Detective Koshien case, where fellow detective Saguru Hakuba accused him of being a poor detective for disrupting a crime scene. Heiji retorted that a detective's primary duty is to prevent death if he can, not simply to preserve a crime scene, demonstrating that his actions are driven by a core motivation to save lives above all else. He is known for his immense pride in his hometown of Osaka, frequently boasting about its culture and cuisine, especially when his friend Conan Edogawa comes to visit.

Heiji's role in the story is multifaceted. He initially appeared in Tokyo seeking to challenge Shinichi Kudo to a deduction battle. After being soundly defeated, he gained a deep respect for his rival. However, during their second encounter, he discovered that the child Conan Edogawa is actually Shinichi Kudo, making him the first person to deduce Conan's true identity. Since this revelation, Heiji has become one of Conan's most trusted allies and partners. He frequently teams up with Conan to solve cases, assists in investigations into the Black Organization, and helps maintain Conan's secret by occasionally disguising himself as Shinichi, even if his Kansai accent often threatens to give him away. Despite his best efforts, he is known for accidentally calling Conan Kudo in public, forcing him to scramble for a cover-up.

One of the most significant relationships in Heiji's life is with his childhood friend, Kazuha Toyama. Their bond was forged in an unusual incident when, as children, he handcuffed 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 closeness, Kazuha later created two identical good-luck charms from the chain links of the cuffs, which both have worn ever since. While Heiji frequently mocks Kazuha for her belief in superstitions and charms, he privately cherishes this talisman. In one instance, after dreaming of Shinichi being murdered, he insisted Conan wear his charm, which ended up deflecting a knife and saving Shinichi's life, though Heiji himself was shot in the stomach without its protection. He is extremely protective of Kazuha and will go to great lengths to keep her out of harm's way, even refusing to let her fall to her death when they were hanging from a cliff, despite being stabbed in the hand by an arrow she used in an attempt to make him let her go. Though they have mutual romantic feelings, Heiji is famously oblivious to romance and to Kazuha's confessions, often mistaking her nervous attempts to declare her love for something else entirely.

In terms of development, Heiji's philosophy on detective work has been shaped by his interactions with Shinichi. He initially seemed indifferent to whether a culprit might commit suicide after being exposed, but after a case where Conan argued that a detective who drives a suspect to suicide is no better than a murderer, Heiji took this lesson to heart. This was evidenced when he intervened to stop a serial murderer from taking his own life, taking a bullet in the process. His relationship with Shinichi has also evolved from a bitter rivalry into a close friendship and partnership, with Heiji even declaring himself to be Shinichi's best friend.

Notable abilities of Heiji Hattori include his exceptional detective skills, which are often on par with Shinichi Kudo's, and his possession of an eidetic memory, allowing him to recall complex sequences and details after only a brief glance. He is a master of kendo, using his skills not only in tournaments but also as a means of self-defense in dangerous situations. He is also a capable motorcycle driver and speaks fluent English, a skill that has proven useful on several occasions.