Kate was introduced in season 7’s “The Power of Three” as the head of UNIT, The United Nations Intelligence Taskforce. The Eleventh Doctor (Matt Smith) takes a liking to her instantly. because she’s the daughter of Brigadier Lethbridge-Stewart (Nicholas Courtney), a beloved long-running character from the classic series.
Kate is smart and idealistic, pushing UNIT from within to be a more peaceful, scientific group. She usually shows up in stories where Earth is under attack, like when Cybermen invade in the season 8 finale. She’s almost always an ally to the Doctor, although at times the two clash due to some ideological differences. Whereas the Doctor is almost always a pacifist, Kate is a little more willing to go to war if she thinks it’s necessary, which leads to a near-falling out in the “Zygon Invasion/Inversion” two-parter in season 9. Luckily she eventually agrees with the Doctor’s pleas for peace, and the two characters remain on good terms.
Kate made a few handful of appearances throughout Chibnall’s era, the latest of which was in the 2021 episode “The Vanquishers.” The fact that she’s appeared so recently on the show means her return in season 14 isn’t too big of a shock, but it’s still significant. Nobody would’ve blamed Davies if he’d decided to bring back Torchwood as the main Earth-bound government force the Doctor had to interact with.