Born into the impoverished Mine family, Miki Kōda was switched at birth with Nozomi Mine by nurse Michiko Nohara, who held a grudge against the wealthy Kōdas. Raised by affluent Daijirō and Sumiyo Kōda, Miki absorbed her mother Sumiyo's lessons that wealth superseded all else, developing a strong sense of entitlement and becoming spoiled and elitist. Coddled and never denied anything, she grew intolerant of the word "no," evolving into an Alpha Bitch who wielded her family's wealth to manipulate situations and people. Her entry into music lacked genuine passion; she relied on financial advantages like private tutors, expensive instruments, and ghostwriters to mask mediocre talent, contrasting sharply with rival Nozomi's success through raw ability and hard work.

Attending the same elite music academy intensified Miki's resentment and contempt for Nozomi's lower-class background. She actively sabotaged Nozomi, framing her for theft with an empty credit card, spreading malicious rumors, and tricking her into missing a critical audition. Miki further bullied Nozomi by making her a personal assistant, subjecting her to verbal abuse and humiliating tasks to reinforce their class divide. Underlying her arrogance were deep insecurities and jealousy. She perceived herself as "Always Second Best" to Nozomi, especially after their teacher Egawa recognized Nozomi's superior talent. This jealousy fueled increasingly underhanded tactics, encouraged by her mother Sumiyo. Egawa noted Miki's profound loneliness, observing she possessed no genuine friends despite her wealth, relying solely on financial influence during hardships.

Michiko Nohara's public revelation of the baby switch on national television triggered Miki's mental breakdown. Learning she was biologically a Mine and Nozomi the true Kōda heir, compounded by the simultaneous death of her biological father Shinsuke Mine, plunged her into crisis. Fearing rejection by the Kōdas, she attempted to flee, overwhelmed by guilt over Shinsuke's death and shame for her past actions toward Nozomi. Following this crisis, Miki underwent significant development. Nozomi intervened, persuading her to reconcile with Sumiyo, who remorsefully acknowledged both girls as her daughters. Miki gradually abandoned her elitist views, beginning to empathize with others and reassess her approach to music and relationships. The anime depicted the families uniting at Daijirō's birthday, acknowledging Miki and Nozomi as equals. The manga presented a darker arc where Miki faced harsher consequences, including public humiliation for plagiarizing Nozomi's work. While the anime softened her bullying and omitted manga-exclusive lows like near-fatal sabotage attempts, both adaptations concluded with Miki embracing humility and forming a tentative bond with Nozomi, symbolizing her departure from initial toxicity.