Hideyoshi Nagachika, often called by the nickname Hide, is a character who serves as the childhood best friend and most important human connection for Ken Kaneki. First meeting in elementary school, Hide approached Kaneki when he was a quiet new student and claimed to have just moved to the neighborhood, a story he invented to give Kaneki a reason to become friends with him. This early act of kindness established a bond that would become central to both their lives.

Physically, Hide is a young man of average height, standing 171 centimeters tall, with a lean build weighing around 58 kilograms. He has short, messy strawberry blonde hair and brown eyes. His wardrobe tends toward bright, casual clothing, often featuring a yellow varsity jacket, and he is frequently seen wearing a pair of headphones around his neck due to his interest in Western music. As the story progresses and circumstances grow more serious, his appearance becomes more subdued, and he later adopts the masked identity of Scarecrow, hiding his features entirely.

On the surface, Hide projects a carefree, cheerful, and sometimes seemingly thoughtless personality. He is outgoing, sociable, and enjoys festivals and lighthearted activities, which contrasts with Kaneki's more reserved and bookish nature. This easygoing exterior, however, belies a remarkably sharp and perceptive mind. Hide is incredibly observant, capable of deducing complex truths from small details in a manner that has been described as Sherlockian. Another character, Nishiki Nishio, notes that while Hide looks like an idiot on the surface, he possesses a keen sense of smell for danger compared to more academically gifted individuals. Hide dislikes formal studying, yet demonstrates exceptional cleverness, having managed to cram for two months to gain admission to the same university as Kaneki.

Hide's primary motivation throughout the story is his unwavering loyalty to Kaneki. He is deeply protective of his friend and consistently works behind the scenes to ensure his safety. Even after Kaneki transforms into a half-ghoul, Hide never wavers in his friendship. He chooses to pretend not to know Kaneki's secret rather than confront him, calculating that this would allow him to offer support without causing Kaneki additional distress. When Kaneki disappears, Hide joins the Commission of Counter Ghoul as an assistant investigator specifically to gather information and find clues about his friend's whereabouts. His investigating hobby, which he describes as playing detective, becomes a crucial tool. He once located the secret base of the Aogiri Tree on his own by placing a tracking device on the ghoul Yamori, a feat that impressed CCG leaders and led to his recruitment.

Hide's role in the story evolves from the lighthearted best friend providing comic relief and emotional grounding to a key hidden player who influences major events from the shadows. He is Kaneki's anchor to humanity and normal life, representing the world Kaneki fears losing. His deductive abilities are demonstrated early when he correctly speculates that the ghoul known as Rabbit and the one called Daughter are different individuals, a conclusion that proves accurate. He anonymously reported the Aogiri Tree base location to the CCG, causing their operation to be moved up by two weeks. His analytical mind and information-gathering skills become so valued that he is appointed as a special investigator's assistant, with one CCG official remarking that the entire organization combined could not gather information as effectively as Hide alone.

The relationship between Hide and Kaneki is the emotional heart of his character. Hide is described as Kaneki's greatest salvation and final refuge. Kaneki internally laments how lonely he is without Hide and thinks of him as a small sun or a savior during his darkest moments. Hide's other relationships include a friendly curiosity toward Touka Kirishima, professional collaboration with investigators Koutarou Amon and Akira Mado, and a tense acquaintance with Nishiki Nishio, whose true nature Hide instinctively senses as dangerous.

Hide's most significant development occurs during the Anteiku raid. Severely wounded, he finds Kaneki hiding in the sewers after a brutal fight. In this moment, he reveals that he has always known Kaneki was a ghoul. He confesses his desire to help and urges Kaneki to eat him in order to regain his strength and fight with all his might to escape. When Kaneki regains consciousness, Hide is gone, listed as missing in action on CCG casualty reports. The ambiguity of his fate persists for a considerable time before it is revealed that he survived, though missing and presumed dead by many.

In the later part of the story, Hide reappears under the masked alias Scarecrow. He cannot speak due to a throat injury and communicates by writing. As Scarecrow, he works to unite ghouls and the CCG, leading the organization Goat to petition for cooperation in saving Kaneki after he transforms into a monstrous creature called Dragon. He collaborates with Akira Marude, Koutarou Amon, and others to separate Kaneki from the Dragon form and recover his original body.

Hide possesses no supernatural abilities or ghoul powers; he is entirely human. His notable abilities are entirely intellectual and perceptive. He has formidable deductive reasoning, powerful logical thinking, and keen perception that allows him to piece together complete narratives from fragmented information. His motor skills and reflexes are also superior to an ordinary person, demonstrated when he once dodged an attacking ghoul and avoided being noticed while placing a tracking device on a dangerous adversary. Despite lacking physical strength or combat ability, his intelligence, determination, and absolute loyalty make him one of the most influential characters in the story. In the end, after years of separation and hardship, Hide reunites with Kaneki, and their friendship endures as a central force in bringing about a peaceful resolution.