she's a victim of parental neglect so bad that they don't celebrate her birthdays, don't call her by name, and don't even seem to care for her existing beyond tuition and living expenses; her mother outright said to her face that she's a mistake and reminds her too much of her father, and that once Mahiru is out of university, she's little more than a stranger at that point. This is actually the catalyst of the plot, as Amane finding her on the swing crying in the rain even so she's very caring to others.