You

A white kitten, given to Granny Weatherwax by Tiffany Aching in a mood of calculated headology. (Wintersmith) Her mother was Pinky, a cat belonging to the Widow Cable, who had a litter of white kittens in the widow’s bed after she passed away. Unable to escape the locked up house, they had to find something to eat. Tiffany dealt with the widow’s body, but the story of the kitten’s first solid meal got around and caused some difficulty with giving the kittens away. (I Shall Wear Midnight; Tiffany Aching’s Guide to Being a Witch)

In the manner of most cats, the name that sticks was acquired almost by stealth, and can be taken as shorthand for "Stop that, You," a milder version of “gerrofoutofityoubugger!”, the pronunciation of Greebo used by most people who have met him.

Tiffany is in some obscure way testing out Granny, a known ailurophobe, with her gift of a cat, viewed in many cultures as the essential accessory for the working witch, or else a natural development in the life of an eccentric old spinster who lives on her own.

Granny has reluctantly accepted the burden of being owned by a cat. (Naïve people refer to this as "cat ownership", which is correct, but then get the relationship of "owned" and "owner" exactly wrong.)

You trained Granny to take her everywhere she goes. As a kitten, You reclined in the warmth and comfort of the space between Granny's tightly bunned hair and pointy hat. As of I Shall Wear Midnight, she drapes herself across Granny's shoulders like a recumbent queen, from which venue she might (rarely) grace someone with the immeasurable honor of blinking at them for a second or two, before losing interest.

You's self-evident mastery of headology has ensured that she eats better than anyone else in the Weatherwax household: whilst Granny and Miss Tick dine on watery bacon soup, You enjoys the luxury of chicken scraps.

You has also, by an act of desperate audacity, earnt the respect (or, more precisely, fear) of Greebo, who now has another reason to hide or not be at home whenever Granny visits Nanny Ogg.

There is a fitting symmetry to one of the most powerful Witches on the disc, a woman dressed in the traditional black, being owned by an all-white cat: this may partly be due to Tiffany's known dislike of the colour black and partly because all-white, the colour of the full moon, is in itself of significance in the arcane world of Witches magic.

It is clear that You is a Real Cat. For further information, refer to The Unadulterated Cat.