Fumino Serizawa's background starts in an orphanage after her parents died in an accident. There, she formed a significant bond with Takumi Tsuzuki. Though separated when Takumi was adopted by Otome Tsuzuki, she deliberately took a part-time job at the Stray Cats patisserie to stay near him.

Her personality exhibits a pronounced tsundere archetype, marked by violent outbursts—often directed at Takumi—like shouting "Die twice!" or "Die ten thousand times!" when angered or embarrassed. This behavior stems from Takumi unintentionally teaching her during childhood to express the opposite of her true feelings. Consequently, she consistently says the opposite of what she genuinely means, a trait only Takumi reliably deciphers. She fabricates unrealistic lies to mask concern for others or romantic feelings, especially when confronted. Her jealousy surfaces visibly when other girls interact closely with Takumi, prompting spontaneous excuses to prevent them from monopolizing his time.

Her romantic feelings for Takumi originate from his consistent protection against bullies during their orphanage years. Despite her affection, she openly questions his obliviousness to her and others' feelings. In panic, she ambiguously confesses both love and hate, leaving Takumi uncertain of her sincerity. Her emotional volatility and difficulty with honesty initially limited her social circle to Takumi and friend Kanae Naruko.

Over time, her relationships expand beyond Takumi and Kanae. She develops contentious but gradually deepening friendships with Chise Umenomori and Nozomi Kiriya, characterized by frequent bickering that evolves into mutual respect. This reflects her gradual emotional opening, though she retains her core defensive mechanisms. Her decisive and strong-willed nature emerges during critical situations, showcasing resilience shaped by her orphaned past and ongoing struggles with vulnerability.