From Discworld & Terry Pratchett Wiki
A graduate of Krull's own college of wizards, and in command of the flying disc that collects Rincewind and Twoflower from the custody of Tethys the sea-troll. She is approximatly 15 years old with exceptionally dark skin (near black). She is a 5th level wizard and Hydrophobe.
She was very worried when she had to capture Rincewind and Twoflower, despite being armed with Ajandurah's Wand of Utter Negativity
“Your effected manner of craven cowardice does not fool me Rincewind. I know you to be a wizard of great power and cunning!”
A strange thing to say, and Rincewind certainly though so. But considering how Rincewind had already rescued Twoflower from the combined Thieves, Merchants and Assassins of Ankh-Morpork, defeated the Old Dark God Bel-Shamharoth and escaped the dragons of the Wyrmberg, then perhaps her concern was very sensible.
Evidently the Krull college of wizards has got the plumbing problem sorted out which prevents Unseen University from recruiting female wizards, as women magic-users are commonplace in Krull. (Either that, or witches are unknown in Krull, or Esk would have been better off applying for membership of this wizarding school!)