Lord Admiral Horatio Nelson was a British admiral who fought in the Napoleonic Wars and a personal friend of the Doctor. He defeated Napoléon Bonaparte's fleet at the Battle of Trafalgar, though it cost him his life. (PROSE: H.M.S. TARDIS, World Game, The Lions of Trafalgar)
The Third Doctor met Nelson at a masked ball in Naples, Italy. Also at the ball were William Hamilton and his wife Emma Hamilton, (PROSE: Danse Macabre) who was Nelson's mistress. (PROSE: The Adventuress of Henrietta Street)
After the Second Doctor was put on trial by the Time Lords, he was sent by the Celestial Intervention Agency with the Time Lord Serena to investigate unauthorised time meddling during the Napoleonic Wars. He discovered that a Player called the Countess intended to kill both Nelson and the Duke of Wellington simultaneously, completely re-writing world history.
In the course of his investigations into the Countess' activities, he met Nelson again when he and Serena thwarted a Player attempt to kill Nelson and Wellington simultaneously during a brief meeting between the two when both were waiting to meet the Prime Minister in 1794, the Doctor and Wellesley disposing of a small bomb that a Player had left in the building. (PROSE: World Game)
Nelson encountered the Second Doctor a second time, although this Doctor was younger than the one the saved Nelson from a bomb. While travelling with Ben and Polly, he materialised on the Victory. Ben, a fellow sailor, was very eager to meet Nelson, and convinced Nelson that he should send the famous battlecry, "England expects that every man this day will do his duty," to the rest of his fleet. The Doctor, for his part, attempted to change history and prevent Nelson from being killed, but he was unsuccessful. During the whole of the encounter, Nelson seemed more amicable to the TARDIS crew than did his flag captain, Thomas Hardy. (PROSE: H.M.S. TARDIS)
For centuries to come, Nelson was remembered and honoured for his victory at Trafalgar. The Trafalgar Square in London was named for the victory, Nelson's Column was in the middle of the square, with a statue of Nelson on the top guarded by four Lion statues pointing in every direction. (TV: The Dalek Invasion of Earth)
On a personal level, he made a powerful impression on Serena despite her brief encounter with him, Serena mourning his death after she learned about his fate at the end of the Battle of Trafalgar. (PROSE: World Game)
Behind the scenes
Nelson was played by Philip Pope in Blackadder's Christmas Carol.