The Marlinspike Hall Cat is a stray cat that lives on the grounds.
The cat is first seen in The Secret of the Unicorn, being chased by Snowy. The chase leads Tintin's model of the ship to be knocked to the floor, causing the mast to break and a scroll to be revealed. This scroll would become the catalyst for the adventure.
After Captain Haddock purchases Marlinspike hall, the cat is seen on the grounds and is frequently bullied by Snowy until the Casta Fiore Emerald, where the pair seemingly bond out of necessity to cope with the loud guests. They are seen sitting side-by-side at the end of the book, as they watch Jolyon Wagg and his company leave.
The cat is then frequently seen with snowy from that point forward, and the pair are apparently friends, though the cat stays at Marlinspike hall, while Snowy journeys the world with Tintin.
The cat is last seen, first being pet by Tintin at the start of The Picaros, then throughout the book with Snowy, and at the end after the adventure is over. It is unknown if the cat was to make an appearance in Tintin and Alph-Art.