Hikaru Gosunkugi is a student at Furinkan High School and a classmate of Ranma Saotome and Akane Tendo. He is one of the least popular students in the school and is portrayed as having no close friends, often remaining in the background and blending into his surroundings. His physical appearance is notably frail and sickly, characterized by dark circles under his eyes which are hereditary, as both of his parents share this feature, a hunched posture, and a generally unwell and gloomy demeanor. His name is a pun; Hikaru is a common Japanese name meaning flashing brilliance, while Gosunkugi can be broken down to mean five-inch nail, a reference to his frequent use of nails in his occult practices.

Gosunkugi is a shy, timid, and socially withdrawn individual. He is acutely aware of his own physical weakness, especially when compared to the numerous martial artists who populate his world, and he realizes he stands no chance against Akane Tendo's other suitors, such as Ranma, Ryoga Hibiki, or Tatewaki Kuno. This insecurity fuels his personality, making him appear antisocial and often resentful of those who are stronger and more confident. Despite his timidity, he possesses a vengeful and obsessive streak, particularly concerning his two main interests: the occult and Akane Tendo. He is a daydreamer who concocts elaborate, often absurd schemes to achieve his goals, although he is prone to clumsiness and his plans frequently backfire on him.

Like many other male characters in the series, Gosunkugi's primary motivation is his deep, unrequited love for Akane Tendo, for whom he has harbored feelings since before Ranma arrived at school. His shyness prevents him from ever directly confessing these feelings to her. Instead, his love for Akane manifests as a bitter rivalry with Ranma Saotome, whom he sees as the primary obstacle to winning her affection. He is determined to find and exploit Ranma's weaknesses in an attempt to humiliate him and remove him as a rival. Gosunkugi's role in the story is largely that of a comedic antagonist and a minor rival to Ranma. Unlike more formidable foes, he does not engage in direct martial arts combat but instead relies on trickery, traps, and unconventional methods to try to get the better of his rival. He is often used as a plot device to create humorous situations, such as when he inadvertently helps uncover a major secret or attempts a harebrained scheme that ends in disaster.

Gosunkugi's key relationships are defined by his obsessions. His dynamic with Akane Tendo is one of distant adoration; he is too shy to speak to her and usually just watches her from afar, though he treasures any small interaction they might have. His relationship with Ranma Saotome is a one-sided rivalry. While Gosunkugi views Ranma as his sworn enemy and constantly tries to sabotage him, Ranma generally does not take him seriously as a threat and often treats him with a mixture of indifference and friendly, albeit accidental, insults. However, despite their rivalry, Ranma frequently and unknowingly saves Gosunkugi from the bullies who often torment him. He also has a transactional relationship with Tatewaki Kuno, who pays him for the photographs he takes of Akane and other girls, hoping the pictures might reveal Ranma's weak point. This arrangement later extends to Kuno's sister, Kodachi. In an anime-exclusive storyline, Gosunkugi briefly falls in love with a kindred spirit named Kogane Musashima, a ghost who shares his interest in the occult, but their relationship ends when she must depart after the summer ghost festival.

Gosunkugi undergoes minimal development throughout the series, remaining a consistently scheming yet ineffective character. One of his most significant storylines is the Neko-Ken arc, where he successfully discovers Ranma's debilitating fear of cats. He then devises an elaborate trap to lock Ranma in a room full of cats, which inadvertently triggers Ranma's Neko-Ken, or Cat Fist, state. While his plan fails to defeat Ranma, he does manage to photograph a moment where the cat-possessed Ranma kisses Akane, which he later uses to torment his rival. On another occasion, frustrated with his lack of physical power, Gosunkugi purchases a high-tech suit of armor called the Warsuit from a mail-order catalog in an effort to compete with Ranma directly. The suit was rigged to explode unless the wearer defeated their stronger opponent. True to form, Ranma refused to lose, and the suit exploded, leaving both of them caught in the blast.

Gosunkugi has a number of notable abilities and quirks, though none of them are related to martial arts. He has a strong interest in the occult and Japanese voodoo, earning him the nickname Voodoo Spike. His trademark is using straw voodoo dolls, which he attempts to nail to trees or other objects to curse his enemies, particularly Ranma, though he is clumsy and often hits his own thumb instead. He is also a proficient photographer, frequently taking candid or voyeuristic pictures of Akane Tendo, which are popular with other male students and which he sells to Kuno. He believes himself to be a master of disguise, though his disguises are almost always easily seen through; a rare exception was when he successfully blended into the bars of a balcony set piece during a school production of Romeo and Juliet to get close to Akane. In the manga, his role is more substantial, while in the anime adaptation, his appearance is delayed until later seasons, and many of his functions are filled by an original character, the ninja Sasuke Sarugakure.