Junichi Kojima is one of the three central characters in the band T.L. Signal, serving as the group's keyboardist. He is close friends with his bandmates, vocalist Kamikurata Kyota and guitarist Hirota Shingo, and the trio forms a tight-knit unit as they pursue their musical ambitions. Junichi is distinct in personality from his more impulsive and dramatic friends. He has a practical and grounded nature, often acting as the responsible heart of the group. In many ways, he is the caretaker, handling tasks like cooking and cleaning, which gives him a stabilizing presence within the band. He is not prone to loud or excessive displays of emotion, preferring to maintain a calm and collected exterior. This reserved demeanor, however, does not indicate a lack of feeling. Beneath his quiet surface, Junichi is perceptive, knowing a great deal about the situations around him, and he cares deeply for his friends. He possesses a subtle sense of humor, often expressed through dry wit or irony rather than overt jokes.

In the context of the story, Junichi and his bandmates are brought into the orbit of a recording company called CosmoPop, which seems to offer them the big break they have always wanted. As Kyota becomes increasingly entangled with the supernatural war that is being waged around them, Junichi and Shingo serve as his connection to a normal life. They are the friends he makes music with, representing his personal dreams and everyday reality that stand in contrast to the growing chaos of the secret conflict involving psychic girls and ancient factions. His key relationships are defined by this friendship and shared goal. Alongside Shingo, Junichi is a pillar of support for Kyota, providing a sense of normalcy and loyalty as their world becomes more complicated.

While Junichi is not a combatant in the psychic war, his primary role is as a musician. His notable ability is his talent as a keyboard player, which is central to the band T.L. Signal’s identity and dynamic. His development throughout the story is tied to the band's journey. As the group is scouted and faces the pressure and allure of a potential career in music, Junichi continues to fulfill his role as the steady and practical member. He represents the grounded, domestic, and reliable element within the trio, balancing the other personalities and keeping the group functional as they navigate both the temptations of the music industry and the supernatural events unfolding around them.