Tsubasa Sonokawa is a 15-year-old high school student with long, straight brown hair and brown eyes, her bangs typically covering her forehead. Standing 161 cm tall and weighing 45 kg, she was born on August 18, a Leo with blood type B. Initially, she develops an intense infatuation with upperclassman Sugiyama, believing he aided her after she fainted from anemia at a train station. This leads her to visit his classroom daily and perform grand gestures, like buying an entire manga volume he mentioned. Her persistence earns her a "stalker" reputation among peers, and Sugiyama rejects her, calling her feelings "heavy" and her actions annoying.

After rejection, she temporarily manages the boys' basketball team when childhood friend Yuri injures her wrist. In this role, she interacts with reserved Aki Hidaka, popular Yūma Toba, and energetic Yoshinari Karasuma. When Aki shields her from a stray ball during practice, her curiosity about him sparks. She later discovers Aki actually helped her at the station, not Sugiyama, shifting her romantic focus to him.

Cheerful, kind, and hardworking, Tsubasa occasionally overreacts or becomes overly intense, making others perceive her as overwhelming. Striving to seem dependable, she consciously tempers her exuberance post-rejection. Growing closer to Aki, she notices his quiet demeanor and appreciation for simple joys like food, associating him with a "sparkling" presence akin to fireflies—a symbol solidified during a training camp moment watching them together.

She navigates a love triangle when Yūma develops feelings after observing her diligence and sincerity. Despite his popularity and interventions, like defending her from jealous classmates sabotaging her cultural festival speech, her attention stays on Aki. Prioritizing managerial duties over personal emotions, she shows maturity. Supportive bonds with Yuri and the basketball team underscore her caring nature and growth from impulsiveness toward measured consideration for others.