Mirror's appearance is almost identical to Yami's in Reigning Shadows, except he wears white and blue, instead of black and red. In addition, his hair and eyes are black and yellow, respectivly, symbolizing a fall into Darkness. He claims that his appearance is part of his curse, and that he had taken the form of the person who would kill him before he completed his task.
A noteworthy fact is that he has two eyes, while Yami only has one.
Little is known about Mirror, except the fact he has no memories from before he was cursed, and that he gets an oddly familiar feeling from Yami.
He is also shown to have some sort of memory loss, as when he woke up and saw Suou, he could not remember who she was at first, even though she was the one who escorted him around after he was met.
In chapter 5 of Reigning Shadows, the "Empress" appears before him. When she asks him whats been happening, he acts confused, and so she calls him a failed experiment. She then does something to him, and reveals that he is actually Experiment Number Delta (END for short). It isn't known what he is exactly, but he doesn't seem to be human.
Also, after the "Empress" fell under Blaid's control, he seemed to regain his sense of self somewhat, for he saved Tamashii from being destroyed by Blaid.
Mirror has shown that he can create a rapier out of thin air to weild as a weapon in a similar manner to Tamashii's scythe. As he has not yet been in an actual fight, it is unknown what powers he possesses.