Red Eyes are a physical characteristic. Scholars believe that the development of red eyes is somehow related to darkness, as the two peoples with the highest instances of red eyes (Veiled Ones and Drow) both inhabit the darkest places of the world. Additionally, albinos may have red eyes due to the lack of pigment in their eyes.
All Veiled Ones have red eyes, as do many inhabitants of the Underdark and Shadowfell. Most Lunarian also have red eyes.
As red eyes are associated with darkness, those who possess the trait (especially non-Veiled Folk or Drow) are said to have been “Kissed by the Shadow.”