Mirrors are enemies found in Grand Bulim Avenue. 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.
After arriving in Grand Bulim Avenue, and dying from spilling an acid bottle all over itself, and continuing the game afterword. The Protagonist arrives back to the main lobby, except only to find a giant lump of flesh where it originally died, along with 2-3 Mirrors in the room. They turn quickly and quickly approach on sight to deal decent damage to the protagonist. Once dead, the protagonist and continue onward.
Although they are originally met within the lobby after acid death. They can and will be encountered again within the same area. Ideally in the dual pathway that leads to the apple puzzle. Where two paths are shown with a picture on different sides. As the paths progress, both pictures change dramatically. Going through the right one will reward the protagonist with ammo and supplies, go through the wrong one will reward Leeches and Mirrors. Of course, the mirrors appear commonly along with other enemies if new game+ is done.