Hanabi Natsubayashi is a supporting character and classmate of protagonist Fumiya Tomozaki in the second year at Sekitomo High School. Her first name, Hanabi, means "fireworks," which is the source of her affectionate nickname "Tama-chan," derived from the traditional Japanese cheer "tamaya" shouted at fireworks displays. Physically, she is notably short in stature with a childish face, featuring wavy, biscuit-colored bob-length hair and mocha brown eyes. She is a member of the school's volleyball club and comes from a family that owns a pâtisserie called "Le Petit Pois," where she occasionally helps out during her free time.

By nature, Hanabi is stubborn, blunt, and fiercely honest, often voicing her thoughts and feelings directly without filtering them or considering the social consequences. This unvarnished straightforwardness frequently leads to friction and conflicts with her classmates, as she is unwilling to go along with things she sees as unjust. Far from being malicious, her honesty stems from a strong personal integrity, as she consistently upholds her principles and refuses to compromise on what she believes is right.

Hanabi's central role in the story is defined by her struggle with social ostracization and her subsequent growth. Her troubles escalate when she openly condemns the bullying of another student by Erika Konno. As a direct result of her public protest, Hanabi herself becomes the primary target of Erika's harassment, which includes having her desk kicked, her pencil leads broken, and eventually her most treasured possession, a plush keychain gifted by her best friend Minami Nanami, being destroyed. Rather than simply enduring the bullying, Hanabi approaches Tomozaki, having recognized the success of his own social transformation, and asks for his help to change herself while still holding onto her core principles. Under Tomozaki's guidance, she practices speaking more cheerfully and works on cultivating more approachable "vulnerabilities" to soften her harsh demeanor.

The most pivotal and defining relationship in Hanabi's life is with Minami Nanami, nicknamed "Mimimi." Mimimi is her polar opposite in personality but persistently befriended her and remains her closest confidante and greatest source of emotional support. Hanabi holds Mimimi in such high regard that she calls her "my hero," crediting her with helping her fit into the class when she first entered school. When Mimimi is suffering in private after losing a student council election, it is Hanabi who provides the comfort and a safe space for her to cry and regain her strength. Conversely, a major reason Hanabi decides to fight back against Erika's bullying is not because of the pain it causes her, but because seeing Mimimi saddened by the situation breaks her heart.

Her relationship with Tomozaki evolves from that of a classmate to a trusted ally and teacher. He records her speech patterns to demonstrate how her tone escalates conflicts and devises a plan for her to practice conversational skills with other friendly classmates like Takahiro Mizusawa and Fuka Kikuchi. Their friendship is solidified when Tomozaki, inspired by a design on his sister's clothing, helps repair her destroyed keychain by adding decorative stitching, a design that is later replicated for their entire friend group to show solidarity. With Aoi Hinami, Hanabi has a respectful but complicated friendship. While she is grateful for Aoi's powerful intervention to stop the bullying, she is also troubled by the cruel and calculated method of social revenge that Aoi orchestrates, expressing that she did not want to see her friend act in such a way.

Hanabi's most significant character development comes during the resolution of the bullying arc. After Aoi Hinami masterfully manipulates the class into turning against Erika Konno, the social pendulum swings to the other extreme, and the class begins to collectively punish Erika with equal cruelty. At that critical moment, Hanabi intervenes, halting the cycle of revenge by declaring that just because Erika did something wrong does not mean they have the right to do the same back to her. She then diffuses the intense situation with a display of intentional clumsiness and a playful pun, "Tama-rrow is another day," showcasing her newfound ability to use her unique charm and personality to de-escalate conflict and foster genuine goodwill. This act of grace and maturity earns her widespread respect and popularity in the class, allowing her to reconcile with former bullies and integrate fully into the social fabric of her school life. Beyond her social acumen, her notable abilities include her skills in confectionery, a talent she has developed from helping at her family's pastry shop.