Eiji Niizuma is a high school prodigy and one of the most exceptionally gifted mangaka in the world of Shonen Jump. Originating from Aomori, he moves to Tokyo following the serialization of his first major work, Yellow Hit. His background is marked by poverty; his family could not afford video games, so from a very young age, he drew manga as his primary form of entertainment, a fact that explains his later dismissive comments about peers who wasted time on games. This early start forged a raw, instinctive talent that sets him apart from nearly every other creator in the series.

In appearance, Eiji has short, brownish-red hair and brown eyes. He is almost always seen in the same purple, long-sleeved shirt and matching jogging pants, and he rarely wears shoes, preferring sandals or going barefoot. He has a famous habit of stuffing his clothing with props related to his current work, such as feathers while drawing Crow or bandages and props to look injured while drawing Zombie Gun. His behavior is as unconventional as his dress. Eiji is relentlessly energetic, often making loud sound effects like "spin spin" or "gas gas gas" while he works, striking weird poses, and contorting his body in his chair. Despite his genius status, he remains humble, referring to his fellow artists as sensei and insisting he is not good enough to judge others work, though he does so when asked.

Eiji’s personality is defined by a pure, almost childlike love for manga. He views his demanding job not as work but as fun, and he reads every single comic published in Jump. He despises sleeping, though when he finally does collapse from exhaustion, he can sleep for up to twenty hours. He refers to himself as an "Eternal Boy," believing this mindset is essential for writing shonen manga. While he appears oblivious to social cues and romantic subplots, lacking success in a romance-themed contest due to having only one crush in elementary school, he paradoxically shows sharp awareness at other times, such as warning a female colleague about the dangers of being alone in a man's apartment. His intelligence is profound; he possesses an almost psychic ability to analyze the manga industry, predicting survey results with perfect accuracy and understanding complex behind-the-scenes editorial politics without being directly told.

His primary role in the story is as the ultimate rival and motivator for the main duo, Ashirogi Muto. From their first meeting, Eiji immediately recognizes their potential, declaring himself their fan and wanting to be their friend. This creates a complex dynamic where he is both a terrifying competitor to surpass and a sincere supporter. He has a special condition from the Jump editors if he becomes the magazine's number one mangaka, he earns the right to end any series he dislikes. However, his motivations are never petty; he genuinely wants to see good manga succeed. This is demonstrated when he immediately joins a creator strike to protest the unfair cancellation of Ashirogi Muto's series, threatening to leave the magazine if the decision is not reversed. Later, when he earns the right to end a series, he surprisingly uses it not to cancel a rival, but to end his own hit series Crow so he can pursue new challenges.

Key relationships define his development. His rivalry with Ashirogi Muto is central, though he is notably closer with the artist Moritaka Mashiro than the writer Akito Takagi. He also builds strong bonds with the other supporting mangaka, such as Shinta Fukuda and Kazuya Hiramaru, often cheering them on and treating them as friends. His relationship with his editor, Yujiro, is one of constant exasperation, as Yujiro struggles to control Eiji's bizarre antics while respecting his unmatched talent.

Throughout the series, Eiji shows subtle but significant development. He begins as an almost untouchable, god-like figure of pure talent. As the story progresses, he experiences failure, such as when his romance manga places poorly in a reader poll, visibly upsetting him. He also learns to collaborate, eventually working directly with Ashirogi Muto on a one-shot, signifying his ultimate respect for them as equals rather than just rivals. This shows a growth from a solitary genius to a cooperative member of the manga creator community.

In terms of abilities, Eiji is virtually without peer. He is both a superb artist and a skilled writer, requiring no team to produce his work. His drawing speed is legendary, capable of producing storyboards that take others days in a matter of minutes. He has an instinctive grasp of storytelling and reader psychology, able to judge the quality and potential of a manga instantly. This analytical ability is so refined that he often functions better than professional editors, diagnosing problems in series that others missed. These traits combined make him not just a character, but the living embodiment of raw, untamed creative genius within the story.