I'm told my voice is usually devoid of emotion, but I somehow doubt that quality makes me unique on this site.
My defining attribute? Depends on who you talk to. My sister, and most of my friends, would probably mention say my automatically caustic responses to everything they say.
People I don't know so well? They might say that I have absolutely no personality, and I'm very boring to be around.
I don't wear any distinguishing clothing, nor do I have any identifying features.