Miya Koshiro is the daughter of the president of Chateau Seika, a prominent sweets and chocolate company. Her family's wealth and influence allow her to leverage their resources, including purchasing the Andorra campus of St. Marie Academy to compete in the World Cake Grand Prix and securing exclusive, high-quality ingredients for competitions. Peers frequently refer to her as "Heiress" due to her status.

Her fixation on Makoto Kashino began when his honest critique of her culinary skills during an enrollment event contrasted with others' flattery motivated by her family's prominence. This ignited an obsessive pursuit of his affection, leading her to view Ichigo Amano as a romantic rival. She repeatedly attempts physical affection toward Kashino despite his rejections and displays jealousy toward anyone close to him.

In competitions, Miya prioritizes premium Chateau Seika ingredients for advantage. During the World Cake Grand Prix, she directed company employees to purchase her frozen fruit cups when initial sales lagged due to weather, securing a win through external intervention. Her reliance on this strategy later contributed to defeats, such as in the semi-finals where Team Ichigo's globally sourced chocolate outperformed her exclusive ingredients.

Her tactics sometimes backfire. In Yumeiro Pâtissière SP Professional, heavy investment in advertising for a sales battle against Ichigo's team gave her an initial lead. However, snowfall reduced final day customer turnout, and the high campaign costs resulted in net losses that contributed to her defeat despite gross sales tying with her rivals. She is partnered with the sweets spirit Marron.

Miya is Johnny McBeal's cousin. Both appear in the second season as part of Henri Lucas's project team alongside Ichigo and Kashino. Her persistence in pursuing Kashino remains unchanged across media, involving continued romantic advances and rivalry with Ichigo.