Iris.
One thing that is unique about me is my eyes. The colour of my iris changes depending on my relationship with someone. My eyes would turn to red when I see someone whom I love very much like my family; blue when I see friends I trust; and brown when it is a stranger.
This characteristic of mine may be quite strange for other people which is why I wear a pair of contact lenses to hide them. Only my family knows this and I plan to keep it that way.