now that I think about it. Its teeth. My main characters tend to have either bad dental work or a specific trait of their mouth area.
Axelle-over bite of dull teeth
Adam: a clear gap
Hila: a canine that sticks out
Charlotte: just awful dental work
Alfred: an overbite with braces
I had to wear brace for 5 years plus 2 years with a retainer, so I guess I don't automatically give my character nice teeth.