Born into the affluent Koda family, Miki Koda (Micci in the Italian dub) lives a life of privilege, her entitlement nurtured from childhood. Unbeknownst to her, she biologically belongs to the impoverished Mine family due to a baby switch performed by nurse Michiko Nohara. Her mother, Sumiyo Koda, actively reinforces Miki's belief in inherent superiority through lavish financial indulgence, exemplified by gifting her an extravagant foreign car for her birthday, cementing Miki's identity as a privileged rival to those of lower status.

Her primary rivalry erupts with Nozomi Mine upon meeting at an elite music academy. Miki relentlessly bullies Nozomi, leveraging her family's wealth for advantages like private tutoring and superior instruments while actively undermining her rival. She engineers public humiliation and professional sabotage, such as framing Nozomi for theft using a depleted credit card and deliberately causing her to miss a critical singing competition slot by spreading false information. This behavior stems from deep-seated insecurity and jealousy, particularly when Nozomi's raw talent overshadows her own despite Miki's resources, fueling a persistent fear of being "Always Second Best."

Miki's world shatters with the public revelation of the baby switch. Learning she is biologically the daughter of the Mines triggers a mental breakdown, intensified by the subsequent death of her biological father, Shinsuke Mine. Fearing abandonment by the Kodas and believing they will favor Nozomi, she flees her home. Her development diverges between media: the anime depicts reconciliation with Sumiyo, who acknowledges both girls as her daughters, signaling potential growth. Conversely, the manga shows a more fractured resolution, with Miki's identity crisis continuing unresolved.

Physically, Miki is distinguished by a distinctive '20s bob haircut with curled accents and sharp tsurime eyes, visually contrasting Nozomi’s softer features and symbolizing her initial antagonistic role.