Natsuki Nakagawa is a second-year student at Kitauji High School during Kumiko Oumae's first year, playing euphonium in the concert band. She ascends to vice president in Kumiko's second year. Her appearance features auburn hair tied in a high ponytail with two strands framing her face and blue eyes, typically seen in the standard Kitauji uniform. Originally depicted with short hair in the novels, her anime design features longer hair in a ponytail.

Initially, Natsuki carries herself with a laid-back and easygoing demeanor, sometimes perceived as lacking seriousness toward practice and music, contrasting with more dedicated bandmates. This calmness stems from a composed nature rather than apathy. Gradually, her personality reveals greater depth: she proves perceptive, kind, and selflessly supportive of peers, offering subtle encouragement. She forms a mentor-like bond with Kumiko Oumae and uses humor to ease tensions, frequently engaging in playful teasing with close acquaintances like Yuuko Yoshikawa.

She graduated from Minami Junior High School alongside Yuuko Yoshikawa, Nozomi Kasaki, and Mizore Yoroizuka. Natsuki joined Kitauji's band due to admiration for Nozomi but failed her initial audition, reflecting early struggles with commitment. Over time, she develops stronger dedication, practicing more diligently and supporting collective goals despite recognizing her technical limitations compared to peers like Kumiko. Her growth is evident in behind-the-scenes efforts, mediating conflicts and creating personalized charms for audition-successful members.

In "Our Promise: A Brand New Day," Natsuki's senior role evolves further. During Kansai competition auditions, she notices first-year euphonium player Kanade Hisaishi intentionally underperforming to prioritize Natsuki's final chance at nationals. Natsuki confronts Kanade and insists on a fair audition, demonstrating her maturity and commitment to meritocracy. Both ultimately pass, reinforcing Natsuki's emphasis on mutual growth.

Her dynamic with Yuuko Yoshikawa features frequent playful arguments and physical affection, including clinging to Yuuko and sharing a kiss. Official artwork also depicts them sharing food under a blanket. A thematic connection exists between their surnames: the kanji for "Nakagawa" (中川) and "Yoshikawa" (吉川) combine to form "nakayoshi," meaning "good relationship."

Personal trivia includes her birthday on June 23, a preference for purple and black, a fondness for bears and spicy food, and dislikes for matcha and anko (red bean paste). Her euphonium case features a yellow bear charm keychain, and she temporarily plays electric guitar during a band event. A spin-off novel, "Watching As You Fly Away," explores her life after graduating from the band.