Harriet Jones
Although Doctor Who fans know who she is, it's worth giving a brief background on Harriet Jones. She was first introduced in Series 1, where she teamed up with Rose and the Doctor to stop the Slitheen. At that point, the Doctor remarked that she would be a three-time Prime Minister. However, the Doctor appreciated her much less in "The Christmas Invasion," where she attempted to represent the planet in negotiations with the Sycorax and then ordered their deaths despite them retreating.
This led to the infamous "Don't you think she looks tired?" line, which ended Jones's political career (and potentially left a power vaccuum for the Master to exploit). Even so, she returned in the Series 4 finale, connecting the Doctor's former companions through the subwave network and facing off against the Daleks to protect the planet.
Altogether, Harriet Jones was in four episodes across Series 1, 2, and 4, though she was also mentioned in Torchwood's first season and Doctor Who Series 10. Like some of the military forces the Doctor frequently ran into, Harriet Jones was a symbol of humanity representing itself against alien threats, rather than letting the Doctor make their decisions for them. While this was not always depicted positively, it added a lot of nuance to the Doctor's self-imposed role as defender of Earth.