Junichi Tachibana is the male protagonist of the story, a second-year high school student whose perspective guides each romantic arc. His background is defined by a painful experience that occurred two years prior to the start of the narrative: he was stood up for a Christmas Eve date by a girl he liked, an event that left him with a deep emotional wound and a significant wariness of love, particularly around the holiday season. This past trauma is the central reason for his initially reserved and cautious approach to romance, often leading him to retreat to the perceived safety of his closet when feeling depressed, a space he has decorated with drawn-in stars and calls his universe.

In terms of personality, Junichi is fundamentally a calm, kind, and patient individual who is generally unselfish and willing to help others in need. He is not a particularly brilliant student overall but does excel in mathematics. However, these positive traits are balanced with notable flaws and quirks. He can be immature, forgetful, and is frequently portrayed as having a pronounced perverted streak, often entertaining exaggerated fantasies and acting on his impulses in ways that range from awkward to brazen. He is also afraid of heights, a phobia that makes him uncomfortable in situations like riding elevators or visiting observatories. His outward demeanor and behavior in romantic contexts are colored by his lack of confidence and difficulty in trusting others' feelings, stemming from his earlier heartbreak. Despite his sometimes wimpy or indecisive moments, he possesses a determined and gutsy side, proving he can go to great lengths for the girl he is involved with.

Motivationally, Junichi is driven by a desire to overcome his past hurt and open his heart to love again, though he is often hesitant and needs encouragement from the heroines to do so. His actions are consistently motivated by a wish to understand and support the girl he is focusing on, even if his methods are occasionally misguided or eccentric. In the sequel series Amagami SS+, Junichi's role evolves from the hesitant suitor to a boyfriend navigating the early stages of a committed relationship, where the narrative explores the new challenges and developments that arise after a couple has formed.

His key relationships are many and varied, as each arc presents a parallel universe where he romances a different heroine. He has a younger sister named Miya who often wakes him up for school. His close friend and classmate is Masayoshi Umehara, with whom he shares many antics. The heroines he becomes involved with include the school idol Haruka Morishima, his childhood friend Kaoru Tanamachi, the shy Sae Nakata, the kouhai Ai Nanasaki, his fellow childhood friend and tea club member Rihoko Sakurai, and the class representative Tsukasa Ayatsuji. Rihoko is noted as the only girl for whom he didn't initially realize his own feelings until the second season.

Throughout the series, Junichi shows development not through a single linear progression, but by revealing different facets of his character depending on the heroine he is with. He remains largely the same person at his core, but the dynamic of each relationship brings out unique changes in him. In Amagami SS+, his development is shown through his efforts to solidify the bonds formed in the first season, facing the more intimate and complex issues that arise in an established couple, such as insecurity, jealousy, and planning for the future. Notably, in some future sequences, he is depicted as becoming a detective, a job that aligns with his previously expressed hope of becoming a police officer.

Regarding notable abilities, Junichi is shown to have developed an interest in photography, often being seen holding a camera in the introduction of Amagami SS+, and he also engages in filming. He has a habit of eating oranges, a possible reference to his surname which means "orange" or "tangerine". His other skills are limited, as it is mentioned he can only cook simple dishes like fried rice.