Hatsuharu Soma is a member of the Soma family who is cursed to transform into the Ox of the Chinese Zodiac. He is a student at Kaibara Municipal High School, attending the same school as Yuki, Kyo, Tohru, and Momiji. He is approximately fifteen to seventeen years old. His appearance is distinctive, featuring shaggy white hair with black roots, which he has had since childhood, and greyish-brown or brown eyes. He typically dresses in punk or gothic-inspired clothing, often wearing black, along with accessories like chains and piercings, many of which he makes himself.

Hatsuharu is best known for his dual personality, which the Soma family refers to as his "white" and "black" sides. In his normal "white" state, he is calm, kind, laid-back, and often appears to be wise but also somewhat confused or absent-minded. When provoked or under extreme emotional stress, he switches to his "black" side, becoming violent, short-tempered, rude, cruel, and even perverted. This split personality acts as a psychological response to the taunts he endured as a child for being the Ox, an animal that was tricked by the Rat in the zodiac legend.

His early backstory involves his resentment towards Yuki, the Rat of the zodiac, whom Hatsuharu initially blamed for the Ox's reputation for stupidity. This resentment ended when Yuki asked him if he truly believed he was stupid, a moment of realization that made Hatsuharu let go of his anger and instead develop a deep and lasting love and loyalty for Yuki. He is fiercely protective of those he cares about, which includes Yuki, his girlfriend Isuzu "Rin" Sohma, and the younger zodiac members like Kisa and Hiro.

In the second season, his primary motivation is his complicated romantic relationship with Rin. Their relationship is secret and tumultuous, as it has drawn the wrath of Akito, the head of the Soma family. At one point, Rin ends their relationship and emotionally withdraws from him, an event that triggers his "black" side, leading him to violently destroy a school classroom, for which he is suspended. He is later seen searching for Kisa and helping her when she runs away from home after being bullied at school, demonstrating his role as an older brother figure to the younger cursed members. He is also protective of Rin, once confronting Akito about confining her.

His role in the story is that of a supportive ally to the main cast and a mentor to the younger zodiacs. He is an upperclassman to Hiro, who admires him and calls him "Haru-nii." He enjoys teasing Kyo in a deadpan manner. He is perceptive about the emotional states of others, using his sharp wit to help them, though his methods can be mischievous. For example, he once coaxed Tohru into using Yuki's first name, which caused Yuki to transform. He also took care of Kisa when she arrived at Shigure's house in her tiger form and stopped speaking due to bullying, reading a troubling note from her teacher and helping her feel safe enough to speak again.

Over the course of the series, Hatsuharu works to navigate the pain of the zodiac curse and his own emotional volatility. His development focuses on learning to cope with his "black" side and his relationship with Rin. After the curse is eventually broken, he reunites with Rin, and they later marry and have twins named Sora and Riku. He expresses a quiet sorrow over the loss of the bond the zodiac members shared, while reaffirming his loyalty to those he loves and working to build a stable life with Rin. One notable ability or trait is his famously poor sense of direction, which is a recurring joke and attributed to being cursed by the spirit of the Ox.