Kunzite is a high-ranking antagonist in the first story arc of Sailor Moon, serving as the fourth and most powerful of the Dark Kingdom's Four Heavenly Kings, the Shitennou. He is presented as a cold, arrogant, and ruthlessly strategic figure, confident in his own strength and given to treating conflict as a game in which he toys with his opponents. His long, flowing white hair, tanned skin, and the distinctive white cape he wears set him apart from his fellow generals, as does his refusal to adopt civilian disguises. He is about twenty-five or twenty-six years old, and his name derives from the pink-to-violet gemstone kunzite.

In his original life during the Silver Millennium, Kunzite was a loyal general of Prince Endymion and one of the guardians of the Earth prince. When the Dark Kingdom reawakens in the modern era, he is reborn as a servant of Queen Beryl, though his loyalty to her is largely compulsory and laced with silent resentment. In some continuities, such as the live-action adaptation, his human identity before being awakened as a warrior is that of a gentle, soft-spoken man named Shin, a personality entirely erased when Queen Beryl turns him into Kunzite. In the manga, Kunzite acts in an advisory capacity to the younger general Zoisite, while in the classic anime adaptation the two share a deep romantic bond that plays a central role in Kunzite's emotional arc. After Zoisite is killed by Beryl for disobeying orders, Kunzite falls into a depression but does not outwardly waver in his commitment to the Dark Kingdom; instead, his grief fuels a single-minded obsession with discovering Sailor Moon's true identity and exacting revenge.

Kunzite's personality is defined by his pride and his desire for autonomy. He dislikes being manipulated, whether by Queen Beryl or by the brainwashed Prince Endymion, and he would rather be his own master than blindly serve any cause. He is perceptive and gifted in strategy, often seeing through the plans of the Sailor Senshi and formulating cunning ambushes. Despite his outward composure, he can become dangerously fixated, and his actions after Zoisite's death shift from serving the Dark Kingdom's energy-gathering mission to a personal vendetta. In the manga, his character takes a different path: he briefly recovers his memories of the past after being exposed to the Silver Crystal's light, leaving him torn between his old loyalty to Endymion and his current servitude to Beryl. Beryl then brainwashes him by embedding a shard of kunzite crystal in his forehead, reducing him to a near-mindless enforcer before he is defeated.

Key relationships revolve around his connection to Zoisite and his interactions with the other characters. In the classic anime, the love between Kunzite and Zoisite is openly affectionate, and Zoisite often clings to him for support. Kunzite's later rampage is fueled by the loss of that bond. In the manga and Sailor Moon Crystal, the idea of a romantic pairing between the Shitennou and the Sailor Senshi is hinted at through an illustration showing Kunzite alongside Sailor Venus, though this concept was never developed in the original story. His relationship with Queen Beryl is transactional: he obeys because he must, but it is clear that he would eliminate her if given the opportunity. He resents being forced to work alongside the brainwashed Endymion and grows suspicious of Endymion's lingering protectiveness toward Sailor Moon. During the final confrontation, he faces the Sailor Senshi alone, and his last words in the anime are a desperate call to Zoisite.

Kunzite's abilities distinguish him as the most formidable of the Shitennou. He wields a shortsword and can manifest an energy blade that he throws like a boomerang, capable of cutting down multiple opponents. He can generate a barrier of dark energy for defense and possesses significant combat flight capability, as seen when he battles Sailor Moon high in the atmosphere in the manga. Uniquely among the Four Heavenly Kings in the anime, he does not rely on pre-existing monsters; instead, he personally transforms ordinary humans into the monsters that carry out his schemes. His strategic acumen allows him to anticipate enemy movements, manipulate situations to his advantage, and remain one step ahead of the Sailor Senshi until his eventual defeat. In both the anime and the manga, he is ultimately killed by Sailor Moon, standing as the only member of the Shitennou to die directly at the hands of the series' heroine.