Eichi Horimiya possesses light brown, wavy hair that proves unmanageable on rainy days, leading him to envy fellow unit member Shu Izumi's straight hair. He has bright green eyes and a relatively tall frame. His unassuming appearance contrasts with a well-defined six-pack, earned through physical labor at a previous job. Naturally gentle, cheerful, and extroverted, Eichi easily befriends people with his friendliness, social skills, and consideration. He consistently prioritizes others' well-being over his own and becomes flustered when Shu compliments him. He acts as a protective older brother figure, especially toward the younger twins Issei and Ichiru Kuga, frequently fussing over their care. His empathetic nature grants him an understanding of perspectives outside entertainment, a quality Shu admires. Raised in a working-class family with two sisters and a pet dog named Chopi, Eichi was often overpowered by his sisters. He attended university and then worked as a television assistant director. On this job, he met actor Shu Izumi and quickly formed a friendship. Visiting the Tsukino Productions building for work, he encountered Shu with the Kuga twins and accepted Shu's invitation to become QUELL's manager. His organizational skills and personality proved effective. Later, prompted by the twins and agreed to by Shu, he transitioned from manager to performing idol due to his dancing abilities and appearance, completing QUELL's formation. Within QUELL, Eichi shares a close bond with Shu, whom he deeply admires. He cooks special meals for Shu during late work hours. He spends significant time cooking and bonding with the twins, showing protectiveness toward the introverted Issei and frequently mediating Ichiru's arguments with SolidS member Tsubasa Okui. Eichi observes similarities between Ichiru and Tsubasa, noting their shared difficulty in expressing feelings honestly. Outside QUELL, he interacts with SolidS members, including occasional cake outings with Dai Murase after discovering Dai's sweet tooth. He maintains polite relationships with members of other units like Six Gravity and Procellarum, who address him formally. His birthday falls at the end of the year, a timing he often forgot due to busy periods before joining QUELL. He volunteers to bake chocolate cake for unit members on Valentine's Day but is easily discovered by the baking aroma. Skilled in cooking, he often uses scissors to cut vegetables and forgoes measurements. He carries candy bars due to past irregular eating habits from his assistant director role. His interests include camping, computers (having built his own), DIY projects, stage art, dogs, and stargazing. He dislikes conflicts, injuries, illnesses, and specific foods like mozuku seaweed. He knows basic sign language learned during university volunteer work and extensively documents unit activities through photos and videos, particularly of the sleeping twins. He and Shu jointly manage the twins' finances, allocating funds monthly to teach responsible spending.