The mirror in the bedroom is just another of Michael's quirks, one that Ken at least enjoys. It's odd to watch himself arching into the touch, knowing Michael is there behind him but being unable to see anything. As long nails drag along his skin, the only visible sign is the raised red trail that is left behind.
Yet he knows the mirror isn't there for his enjoyment, just as he is not. He, like everything else in the mansion, belongs to Michael.
Michael watches the mirror, watches his face, for every line or emotion that plays out upon the reflective glass. A painter studying his subject as he slowly peels off each layer that hides what he is looking for.
Ken watches as well, while Michael takes him or sometimes just painfully strokes him off. He becomes nothing but a puppet played well by those invisible hands pulling on strings that leave no sign. His body moves against nothing and it's only when Michael desires to feed that there is any sign he is not alone. The dark crimson of his blood as it clings to Michael's lips the last needed sign before his own desire is fulfilled.
Ken knows he's the only one to see this silver masterpiece, no one else is given to observation like Michael does with him. It warms his blood just that much more to know this, just another sign of his place in Michael's heart.