An Aes Sidhe Witch is an Aes Sidhe that was once a cat but was blessed with human form and magic. However if an Aes Sidhe witch uses too much of her powers too quickly, she will be turned back into a cat permanently - as happened to Shalott.
An Aes Sidhe Witch is able to morph from their human form to their cat form at will (unless stuck in their cat form through overuse of powers). When human their colouring will generally reflect their cat form, so a black-furred cat will have black skin and black hair as a human[Pan 1](citation needed).
Aes Sidhe Witches fit into Aes Sidhe society.
Once reverted to cat-form for eternity, Shalott retired to Albion. (citation needed)
"The Aes Sidhe witch in NeS is based upon the mythological Cat Sidhe of Scotland, in which the cats are witches who can transform into a cat nine times - at which point they're stuck as cats forever. This gave birth to the myth that cats have nine lives[Ext 1]. I altered it to be that an Aes Sidhe may use up too much magic and thus be turned into a cat forever." - Britt the Writer