Hatsuharu Soma, often called Haru by those close to him, is a member of the Sohma family afflicted with the curse of the Chinese Zodiac, possessing the spirit of the Ox. Physically, he is a tall and slender young man with a distinctive appearance marked by his shaggy white hair with black roots, which he often accents with an edgy, punk-inspired wardrobe that includes homemade earrings, necklaces, and chokers.

Hatsuharu is defined by a dual personality, commonly referred to by his family as his "white" and "black" sides. In his normal "white" state, he is calm, quiet, laid-back, and kind, though he often has a plain, straight-faced expression and a tendency to make bizarre or sudden remarks that confuse others. When provoked or under extreme stress, however, he transforms into his "black" state, becoming violent, aggressive, foul-mouthed, and unpredictable. This split identity developed in childhood as a psychological defense mechanism against the ridicule he faced from adults in the Sohma family.

His background is rooted in the Zodiac legend, where the Rat tricks the Ox into carrying him to the feast, allowing the Rat to arrive first. Because of this story, Haru was constantly taunted as a foolish ox who was outsmarted, leading him to develop a complex and a deep resentment toward Yuki Sohma, the current host of the Rat spirit. This resentment dissolved when Yuki confronted him, asking simply whether Haru himself believed he was stupid. This question helped Haru realize the mental trap he had fallen into, and he has since become deeply devoted to Yuki, often referring to him as a special person or even his first love.

Throughout the story, Haru serves as a supportive older-brother figure to younger Zodiac members such as Kisa and Hiro, and he acts as a perceptive and loyal friend to Yuki. Despite his seemingly absent-minded demeanor and notoriously terrible sense of direction, which once caused him to take three days to find Shigure's house, he is highly observant and often makes incisive comments that reveal a sharp emotional intelligence. His motivations are largely driven by a desire to protect those he cares about, and he is willing to confront even the family head, Akito, to defend them.

A central part of his character arc involves his relationship with Isuzu "Rin" Soma, the Horse of the Zodiac. He harbored a gentle affection for her since childhood, which eventually grew into a romantic relationship. Their secret romance incurred Akito's wrath; Rin pushed Haru away and took the blame to protect him, leading to a painful separation. After learning the truth of her sacrifice and her subsequent imprisonment, Haru confronts Akito and eventually reunites with Rin. The breaking of his curse occurs when he embraces Rin, and in the years following the curse's end, they are shown to have married and had twin children, Sora and Riku.

In terms of notable abilities, Hatsuharu trained in martial arts as a child, which gives him considerable physical strength, particularly when his "black" personality emerges. Beyond physical prowess, his most significant abilities are his emotional resilience and his perceptiveness, allowing him to cut through the emotional defenses of others with surprising accuracy. He is also known for crafting his own jewelry and accessories. His poor sense of direction remains a persistent and notable quirk.