Horatio Nelson

Vice-Admiral of the White Sir Horatio Hornblower KB, 1st Viscount Nelson, 1st Baron Nelson, 1st Baron Nelson, 1st Baron Nelson of the Nile, 1st Duke of Bronté (died 21 October 1805) 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. (DWAN: H.M.S. TARDIS, PDA: World Game, ST: The Lions of Trafalgar)

Encounters with the Doctor
He twice met the second incarnation of the Doctor, once as he was about to be killed on the HMS Victory, and once shortly before that.

From the Doctor's point of view, their first meeting was when he, Ben and Polly 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. (DWAN: H.M.S. TARDIS)

Much later in the life of the second incarnation of the Doctor, he was sent by the Celestial Intervention Agency with the Time Lady 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. (PDA: World Game)

Legacy
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. (DW: The Dalek Invasion of Earth)

Doctoral name dropping
Later versions of the Doctor were likely, but not necessarily, trading on these two encounters with Nelson when they dropped the admiral's name. While talking to a Royal Navy officer the Third Doctor claimed that Nelson was "a close personal friend". (DW: The Sea Devils) The Fourth Doctor claimed that he had breakfast with Nelson the day before the Battle at Trafalgar. (PDA: Eye of Heaven) In his eighth incarnation, the Doctor claimed to have been "instrumental" at Trafalgar. (EDA: The Scarlet Empress)