|“||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.||”|
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 bloodline, tend to remain in the bloodline for generations.
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".
Part-Sidhe by Blood
Sidhe can procreate with both humans and other fey, resulting in mixed bloodlines, the likes of which are usually shunned by the Seelie.
Primarily, it is possible for humans, other fey, and part-sidhe to temporarily become fully Sidhe via Branwyn's Tears. However, it has also been shown that humans and even abstract concepts can permanently become at least part-Sidhe.
For example, Frost was never born but made from hoarfrost by the love of the human girl, Rose. He was the original Jack Frost. Frost was once part of the Seelie Court, but having been created and not born sidhe, he was never sidhe enough for the glittering throng. While the Seelie Court's powers waned due to the lack of human worship, his power remained as humans continued to remember and believe in Jack Frost.
Humans have been shown to become permanently fey through exposure to wild magic, either ambient or directed. Ringo became part-fey (though not Sidhe) after a group of visiting sidhe from the Seelie Court granted his wish and poured wild magic into him (explained in A Kiss of Shadows). Gilda became sidhe after wishing on naturally-occurring wild magic (explained in Divine Misdemeanors).
"To make a full human into a part fey, you have to pour wild magic, pour power, into them and it is the human's will or desire that chooses the shape of that magic." ~ Merry Gentry in A Kiss of Shadows (Page 108)
"There is always something twisted inside the minds and bodies of humans who are given access to the wild magic of faerie" ~ Doyle in Divine Misdemeanors (Page 17)
Note: Alistair Norton was a human wizard who magically seduced part-fey women in order to steal their powers through a ritual.