Eiji Nobunishi is a supporting character in Battery the Animation who serves as a key antagonist, particularly in the early episodes. He is a third-year student at Nitta East Middle School and holds the position of vice-captain of the school's baseball club. Beyond his athletic responsibilities, he is characterized as an honor student who also performs the duties of a disciplinary committee member. His physical appearance includes black hair cut to ear-length and black eyes, and he is typically depicted wearing glasses.

As a senior student, Nobunishi embodies a specific type of hardworking, rule-abiding individual. He is a diligent student and has worked his way up through the ranks of the baseball club to become a starting player. His primary motivation for joining and persisting with the baseball club is pragmatic rather than passionate. It is revealed that his goal is to use his position on the team to build a good reputation that will benefit his entry into high school, and he has little genuine love for the game of baseball itself. This utilitarian view of the sport is a core component of his personality and is the source of his major conflict with the protagonist, Takumi Harada.

Nobunishi's role in the story is that of an adversarial upperclassman. He takes an immediate and intense dislike to Takumi, a prodigiously talented first-year who joins the team. His resentment stems from jealousy; he cannot accept that a younger newcomer, who seems to have a dismissive and insolent attitude, has instantly achieved a level of recognition and skill that Nobunishi himself has spent years struggling to obtain. This animosity escalates dramatically when Nobunishi, feeling threatened and humiliated, assaults Takumi after a practice session. This violent act is a pivotal moment in the narrative, leading to severe consequences for the entire baseball club, including their suspension from an upcoming tournament. Despite attempts by the school administration and even his own coach to downplay the incident, Nobunishi shows little remorse for his actions, primarily viewing himself as the victim of an unfair situation caused by the disruptive newcomer.

In terms of key relationships, his primary dynamic is with Takumi Harada, whom he bullies both psychologically and physically. He also acts in opposition to Coach Tomura, whose coaching philosophy emphasizes teamwork and clashes with Nobunishi's self-centered approach to the sport. Nobunishi is shown to have accomplices within the team who assist him in his bullying, and the third-year captain is later seen pleading for him to remain on the team, highlighting the complex and, at times, distorted social hierarchy within the club.

Nobunishi's development arc is one of self-destruction rather than growth. When his commitment to the sport as a purely transactional means to an end is undermined by Takumi’s natural talent, he is unable to adapt. His jealousy and rigid sense of entitlement lead him to commit an act of violence that ultimately backfires, jeopardizing his own goals and his team's season. In the aftermath, he quits the baseball club, a decision framed as a psychological breakdown that serves as a cautionary parallel for Takumi, illustrating what can happen when the values a player has placed their trust in stop providing rewards. His notable abilities lie in his role as a dedicated, hard-working player who has earned his spot through discipline, but he is consistently outshone by the pure, innate talent of the protagonist.