Kathleen is an Irish heifer (a type of cow), owned by an Irish farmer.
When Thunderbird 1 vaned down to a landing outside Kathleen's barn one day, Scott Tracy made her move over, to make way for his Mobile Control Centre. All the while when he was searching for the missing Fireflash 4 jetliner (presumed to have crashed somewhere off the Irish coast), Kathleen maintained a certain wary eye on the stranger who'd commandeered her barn.
But it seems that Kathleen had the last laugh (or is that the last "Moo"?), after Scott finally returned home to Tracy Island: "Sorry I'm late, folks - I had to milk the cows before I could take off!"
Kathleen appears in the episode Operation Crash-Dive.