Heather Sellers doesn’t know who she is. I mean she knows, but she can’t recognize her own reflection in the mirror. Or the faces of others. She suffers from Prosopagnosia, or face blindness. It’s not as uncommon as you might think, but most often it is mild and only slightly aggravating.
Acquired prosopagnosia often results from head trauma, stroke, or developmental prosopagnosia (as Heather Sellers has) seems to genetically determined and begins at a young age, before the development of normal facial recognition abilities. How do these folks recognize family and friends? And themselves? Usually by some combination of voice, clothing, hair style, mannerisms, walking gate, body language, or some combination of these and other cues.
Here is an interesting video of someone who has this malady.