Gertrude Fairweather is a woman living in a house just west of Varrock. She is the starting point for both the Gertrude's Cat and Ratcatchers quests. Additionally, once your cat has become Overgrown, she will sell you kittens for 100 but keep in mind you can only have one kitten at a time. The colour of the kitten is random unless you have an activated ring of charos, then you will be able to charm Gertrude into letting you choose the colour of the kitten.
She has four sons: Wilough, Shilop, Philop and Kanel.
Once a player catches 100 rats with their cat, she'll award them with the cat training medal. Note that this must be done before their cat reaches overgrown stage, or they'll not get the reward.
Gertrude is the solution to the beginner anagram clue: RUG DETER.