Thunderbird S, with "S" standing for "Shadow", is a top secret International Rescue stealth aircraft. Coloured black and piloted by Kayo, Thunderbird Shadow is used for recon missions and for keeping the security of Tracy Island. Although considered an official Thunderbird, it is not used primarily for rescue missions like the main five, and is instead used for missions dealing with stopping villains and keeping International Rescue secure, much like Lady Penelope in FAB 1.