If you can kill sidhe, do it, but don't fuck them up and leave them alive, because they never forget, and they never forgive.
— King Kurag in Seduced by Moonlight, chapter 5

Full blooded sidhe usually possess tri-colored or triple irises, and luminous, occasionally glowing skin. Sidhe genetics, up until the last few hundred years, had always bred true, and once in a blood line, tend to remain in the blood line for generations.

By John Duncan

Riders of the Sidhe by John Duncan

The self proclaimed royalty of the fae, over the centuries, they separated into two courts; the Seelie, the court of light and illusion, and the Unseelie, court of air and darkness.  Sidhe possess some of the most powerful magics of all the fae. In past centuries, many were powerful enough to be called gods by the mortals around them.

The sidhe have distinct tribes, ruled over by kings and queens in each territory. It would seem that the social order of the sidhe corresponds to the old aristocracy of ancient families, which is a reflection of the ancient Celtic caste system. Many historically refer to the sidhe as simply "the gentry".