There are characters who start off as playing minor roles who later turn out to be Breakout Characters, Ensemble Dark Horses, Ascended Extra, and any number of Fan Favorites.
And then there are characters who are such incidental background characters that you don't even realize that there was a time they didn't exist before then. Blondie's pet Daisy (and her puppies) is a prime example. Today's modern readers probably aren't even aware there was a time she was with child. As far as they're concerned, she's always been the family pet doing silent commentary.
While there's now a tag for the cat's infrequent appearances or references, the number of strips since 2000 is surprisingly low for what's essentially a non-speaking part in a comic that's pretty much all about snarky dialogue. So it's probably not such a surprise the cat doesn't play a larger part.
Sally Forth comic story arcs generally run the gamut from anywhere from three to four comics to the full six in a week. So this is one of the rare instances where we have an arc that lasts a good two weeks. I can't recall any other instance where a pivotal addition had such length added to them. If there were any others, I obviously don't know of them. After a week of persuasion and build-up, we finally get around to purchasing said cat, with predictable results.
Nowadays, the cat goes by the generic name 'Kitty', but back then, Hillary chose an entirely different more interesting name for the cat:
And with their latest addition to the Forth family assured, the cat is free to make her presence known... whenever anybody notices her long enough to include her into the conversation.