Mirrors are enemies found in the Grand Bulim Avenue train station. They are fast creatures that confront the player with a distorted image of themselves. However, they can be killed quite easily, but hardly without inflicting damage first.
The Mirrors appear to be manifestations of the Protagonist's issues with self-image and weight.
Another possible explanation might be that the Mirrors are manifestations of how the Ghost Girl saw herself, ugly and twisted.
Since it's implied through notes left throughout the Royal Phantome Subway and the Grand Bulim Avenue train station that the Ghost Girl suffered from severe issues with her weight and body image, it could be taken that, along with the Leeches and the Ghost Trains, the Mirrors are another symbolic demon of the Ghost Girl, taken form and beset upon the Protagonist.