Within the series narrative, Kate Lethbridge-Stewart serves as the head of Scientific Research and later the commander-in-chief of the Unified Intelligence Taskforce (UNIT), an organisation that defends Earth from alien threats.

Kate is one of the longest-running recurring characters in the show's revived run, appearing alongside the Eleventh, Twelfth, Thirteenth, Fourteenth, and Fifteenth Doctors.

However, a shared experience fighting the Great Intelligence alongside the Brigadier and Sarah Jane Smith allows her to reconnect with her father and develop a closer relationship.

She is now the head of Scientific Research for UNIT, opting to lead the military organisation towards a more science-based approach, and has dropped Lethbridge from her surname as to not receive any special favours.

[17] She also appeared in the audio drama Past Lives, the first of a seven-part series produced for the programme' 60th anniversary entitled Once and Future, in which Redgrave starred opposite Tom Baker as Fourth Doctor, Sadie Miller as Sarah Jane Smith, and Rufus Hound as the Monk.

[18] The character of Kate Lethbridge-Stewart was initially created by Marc Platt for the spin-off drama Downtime, which was released direct-to-video by Reeltime Productions in September 1995 during the period that Doctor Who was off-air.

Cressman reprised her role as Kate for a sequel spin-off drama entitled Dæmos Rising (2004), which was released direct-to-DVD by Reeltime Productions in March 2004.

[22] Once the script was in production, Chibnall, primarily a fan of the television series, was surprised to discover that the character of Kate already existed in non-television spin-off media, which he had been completely unaware of.

"[23] Moffat decided to bring back Kate for the programme's 50th anniversary special "The Day of the Doctor" (2013) and additionally introduced Ingrid Oliver as UNIT's scientific advisor Petronella Osgood.

[24] Kate and Osgood continued as recurring characters for the remainder of Moffat's time as showrunner, appearing in four episodes of the programme's eighth (2014) and ninth series (2015) alongside Peter Capaldi as the Twelfth Doctor.

[25] Redgrave was intended to appear in the tenth series episode "The Pyramid at the End of the World" (2017), but her character was replaced by Colonel Walsh in the final script due to her prior filming commitments for the BBC drama Holby City.

[25] Following Russell T Davies' return as showrunner the following year, Redgrave reprised the role of Kate, now using the full surname Lethbridge-Stewart, in the 60th anniversary special "The Giggle" (2023), which starred David Tennant as the Fourteenth Doctor.

[32] Jemma Redgrave and Alexander Devrient are expected to reprise their respective roles as Kate Lethbridge-Stewart and Colonel Ibrahim for the spin-off, alongside Russell Tovey and Gugu Mbatha-Raw as new characters.

[33] Following her debut in "The Power of Three", Patrick Mulkern of Radio Times welcomed Kate Lethbridge-Stewart as "a wonderful addition" to the show and enthused about Jemma Redgrave's "class-act casting," describing her as "elegant, earnest, warm" and requesting that her character come back regularly.

[21] Similarly, Digital Spy reviewer Morgan Jeffery felt that Redgrave "immediately endears herself" as Kate and praised the "lovely moment" that was her reveal as the Brigadier's daughter.

[34] Neela Debnath, writing for The Independent, thought that the re-introduction of Kate as head of UNIT was "a wonderful way of preserving the Doctor’s relationship with the organisation and Brigadier Alistair Gordon Lethbridge-Stewart."

The Daily Telegraph reviewer Gavin Fuller agreed that her leading her father's organisation was a "lovely touch" and praised Redgrave's portrayal, stating that he would like to see the character return in the future as well.

[37] For her review of the same episodes, Kaite Welsh of IndieWire felt that the tension between Kate and the Doctor due to their differing approaches was "handled admirably in this two-parter" and added that "Redgrave is at her best when there’s a glint of the zealot about her.