Haruhi Teramitsu forms one half of the Teramitsu twins, born in England to a British father and Japanese mother. He shares mirrored physical traits with twin brother Yuduki, including a mole on the opposite side of his neck and contrasting blond hair against Yuduki's black. Childhood illnesses frequently confined Haruhi, restricting his social world primarily to his brother. Their family life unraveled through their mother's chronic hospitalization and death, their father's abandonment, and a subsequent move to Japan with their grandfather after their grandmother's passing. Following their grandfather's death, the twins entered an orphanage where they bonded closely with the director, whom they called "grandma."

Haruhi displays an extroverted, attention-seeking personality, often proudly showcasing achievements and craving praise. He habitually overexerts himself when entrusted with tasks and reverts to English under extreme stress. His interests encompass fashion, shopping, stage comedies, and the mascot brand Oshiriz—a fascination rooted in childhood scarcity of Japanese goods. He also exhibits a distinct habit of evaluating people's buttocks upon first meetings. Beneath his cheerful exterior lies deep loyalty, particularly toward Yuduki.

His bond with Yuduki remains intensely close yet complex. Haruhi relies on Yuduki for emotional support, a dependency forged during illnesses, and demonstrates fierce protectiveness. Historically, he diverted romantic interest from Yuduki to himself as a shielding tactic—a behavior reignited when a female celebrity pursued Yuduki, prompting Haruhi to deploy calculated interventions. This overprotectiveness sparks friction as Yuduki seeks independence beyond their twin identity. During a camping trip, Haruhi neglected sunscreen, causing sunburns that ignited an argument where he expressed lifelong gratitude and protective intentions. They later reconciled with mutual apologies.

Professionally, Haruhi debuted in the idol unit KiLLER KiNG alongside Yuduki, Akane Fudo, and Miroku Shingari after three years of joint training. Despite initial disappointment at not being selected for B-PROJECT's debut, he committed fully to his unit. His dedication surfaced during a collaborative live with THRIVE when he supported Miroku's struggle to retain a center position by practicing alternative choreography and advocating teamwork.

Haruhi's relationships extend beyond his twin across media. He shares a mentor-like dynamic with Goshi Kaneshiro as part of the "Returnee Trio," expressing admiration through cooking curry for Goshi, borrowing his clothes, and aspiring to share adult experiences like drinking. He gradually forms a genuine connection with Tsubasa Sumisora, B-PROJECT's A&R staff, shifting from lighthearted flirtation to deeper reliance—particularly during twin conflicts. A library outing highlighted this evolution: Haruhi sought Tsubasa's book recommendations but prioritized Yuduki's choices upon noticing his brother's quiet disappointment, displaying newfound sensitivity.

His character growth peaks in Ryusei*Fantasia when protective instincts clash with Yuduki's desire for autonomy. After discovering Yuduki confided their orphanage past to Tsubasa, Haruhi reacted angrily, viewing it as private. Tsubasa's intervention helped him comprehend Yuduki's perspective, prompting Haruhi to reassess his approach. This shift extended to his feelings for Tsubasa, evolving from superficial advances to sincere interest.