Junichi Baba, known as B.J., is a student from Iwanagi Junior High School who appears in the story of The Law of Ueki. He is a second-year student with a distinctive appearance, characterized by his brown, dreadlock-styled hair and a preference for casual hoodies, reflecting his deep immersion in hip-hop culture. His genuine love for hip-hop music is a core part of his identity, initially drawing him towards others who seem to share the same interest. Beyond music, he has a fondness for panna cotta.

Personality-wise, B.J. presents a contradictory mix of traits. On one hand, he can be friendly and sociable, quickly taking a liking to the protagonist, Ueki, when they first meet on a train. On the other hand, he has a notable and persistent tendency to lie, especially about his own identity. This habit is most dramatically displayed when he impersonates the extremely powerful and feared Robert Haydn to intimidate other power users. His behavior also includes a lecherous side, as he often directs inappropriate actions towards the female lead, Ai Mori, such as using his power to flip her skirt or taking other liberties that promptly earn him a physical reprimand.

B.J.'s primary motivation and role in the story is that of a catalyst and a minor antagonist turned ally. His involvement begins with his deception, but when his own ability is discovered, he challenges Ueki to a battle. Although he is defeated, the encounter ends on friendly terms. However, this brief conflict serves a greater narrative purpose: his impersonation of Robert Haydn attracts the attention of the real Robert, who appears on the scene and soundly defeats B.J. This event serves as the critical point of contact that introduces Ueki to the existence and threat of Robert Haydn, setting a major story arc into motion. Later in the series, B.J. reappears not as an enemy but as an acquaintance of Ueki, even lending indirect assistance during a crucial battle by distracting Ueki's opponent, which helps prevent Ueki from being crushed by a giant doll.

Regarding his abilities, B.J. is a power user with the talent to turn coins into whirlwinds. While he can generate these whirlwinds, his combat skills are not particularly formidable. Ueki defeats him without much difficulty, and the real Robert Haydn defeats him even more swiftly and decisively. Despite the high potential of his power, his character does not undergo significant development or power growth; his importance remains tied to his role as an early obstacle and a narrative device. His key relationships are primarily with Ueki, who becomes something of a friend, and with Ai Mori, who is typically the target of his unwelcome advances. Overall, B.J. serves as a colorful, comedic, and slightly troublesome figure in the early part of the story, whose actions have lasting consequences for the main characters.