Mia Costa is a high school student from Italy. She has dark blonde hair and blue eyes, and she often keeps her hair in a bun. While her mother was pregnant with Mia, a man with a crescent moon mark contacted the family to arrange for Jim Gore to accompany her mother to a hospital. This early connection foreshadows her later entanglement with the Moriarty family’s schemes. Mia is gentle and emotionally perceptive, remaining remarkably calm even when surrounded by chaos. She meets Ron Kamonohashi and Totomaru Isshiki at the Auberge Reunion, where she impresses them with her Japanese language skills. As the story unfolds, she becomes a key person whom Ron strives to protect, as she is drawn into the dangerous machinations of the Moriarty family. Her role is that of a supporting character in the second season, but her connection to the criminal underworld and her calm demeanor make her an important figure in the narrative. Further details about her abilities or personal development are not extensively covered in available sources.