Brains (born 14 November 2001/2040) is International Rescue's chief engineer and designer/inventor of many of the secret organization's cutting-edge life-saving machines, most notably the five Thunderbirds: Thunderbird 1 (rapid-response swing-wing rocket plane), Thunderbird 2 (heavy-duty cargo-carrier), Thunderbird 3 (single-stage-to-orbit spaceship), Thunderbird 4 (deep-diving mini-submarine) and Thunderbird 5 (Earth-orbiting space-station and IR's chief listening post).
He resides on Tracy Island, home of the Tracy family (and IR's hidden headquarters), located somewhere in the greater South Pacific.
Brains' keen intellectual aptitude was evident even at an early age; but tragedy intervened when he was 12 years old - a freak storm destroyed his family's Michigan home, and both of his parents perished. The young genius was then adopted by a Cambridge University professor, who encouraged him to further his studies.
Enter International Rescue...
Jeff Tracy - himself no stranger to family tragedy and loss - had conceived the idea of of a secret organization that could save lives when all other conventional means failed. However, he as yet lacked just the right person with the technical know-how and the overarching vision that could give tangible reality to his hopes...
While attending a major scientific gathering in Paris, he came across a certain young speaker (just about the same age as his own sons) giving a presentation at a small cultural hall well away from the main conference venue. Here was the very man he's hoped to find - and afterward he took Brains aside and outlined what was essentially a job-offer unlike any other. Brains, for his part, saw in the former astronaut and retired multi-millionaire businessman a fellow visionary, one who would engage his genius in the service of humanity. A deal was struck, Brains joined Jeff and the rest of the Tracy family on Tracy Island...and the rest would become history, beyond imagining...
So, what's in a name? "Brains" is in fact his nickname (his real one is unknown), and it's one he's constantly living up to. He's happiest when he's trying out new ideas, and inventing new things...from the latest rescue gadgetry for one or another of the Thunderbirds, to his ongoing efforts to teach his intelligent robot, Braman, to play chess, a popular Tracy Island pastime...to arranging a certain very special surprise for his friends, just in time for Christmas. Other favorite hobbies include mathematics, especially trigonometry (!), and scuba-diving.
When his expertise is required in the field, he'll accompany the Tracy brothers to the danger-zone (Pit of Peril being a good example); and he periodically visits Thunderbird 5, for inspections and equipment upgrades aboard the space-station.
Although he has his own IR uniform (his sash-color is brown), he usually prefers civilian-clothes for his sorties aboard Thunderbirds 2 or 3 (he does wear uniform, though, in the feature-film Thunderbird 6).
He also "keeps his hand in," so to speak, in developing revolutionary new aircraft for organizations other than International Rescue - albeit always under an alias - such as Skythrust (Alias Mr. Hackenbacker) and Skyship One (Thunderbird 6).
Brains on Thunderbirds Wiki: https://thunderbirds.fandom.com/wiki/Brains