Harry Sullivan

Surgeon Lieutenant Harry Sullivan, a Royal Navy surgeon in the employ of UNIT, was also for a time, the companion of the Fourth Doctor. He later worked for NATO and MI5.

Career with UNIT
He first encountered UNIT while working in the Royal Navy, during which time the Third Doctor had briefly left Earth in his TARDIS to Peladon. (PDA: The Face of the Enemy, DW: The Curse of Peladon).

Later, after he had joined UNIT and the Third Doctor was apparently in a pre-regenerative coma, the Brigadier summoned "Dr Sullivan" to help see him through the process. Sergeant Benton, however, woke the Doctor by offering him a cup of coffee. The Brigadier told him not to bother. (DW: Planet of the Spiders)

After confronting the Great One on the planet Metebelis III, the Doctor disappeared. Sarah Jane believed he had died until after three weeks, the Doctor's TARDIS re-appeared in his laboratory at UNIT HQ. The Doctor had received a near-fatal dose of radiation and would have to either regenerate or die. (DW: Planet of the Spiders) Harry Sullivan arrived on the scene just moments after the Doctor regenerated into a new form (DW: Robot)

Adventures with the Doctor
Harry Sullivan, at very first much confused by the Doctor, trailed him and his companion Sarah Jane Smith. After the defeat of the robot K1, Harry found himself in the Doctor's TARDIS with them. (DW: Robot) The three went first to Space Station Nerva and were pitted against Wirrn (DW: The Ark in Space) and then by transmat from the space station,  travelled to an uninhabited far future Earth in the same time period where they faced the Sontaran Styre. (DW: The Sontaran Experiment) The transmat next took them not back to Space Station Nerva, where they had left the TARDIS, but to Skaro during the time of the creation of the Daleks on a mission given them by the Time Lords (DW: Genesis of the Daleks) Via time ring, they traveled back to Nerva, but the time ring had transported them to a much earlier time period. (DW: Revenge of the Cybermen)

Back in the late 20th century, Harry joined up with UNIT again to defeat the shape-changing Zygons and their giant dinosaur-like beast, the Skarasen, in Scotland. The Zygons captured Harry and one assumed his form. He decided to go home to London by train rather than by TARDIS. (DW: Terror of the Zygons)

A short time later, Harry helped the Doctor against the Kraals. (DW: The Android Invasion)

Post-UNIT career
In the early 1980s, Harry went to work at NATO. (DW: Mawdryn Undead, TC: Harry Sullivan's War) By 1997 he had gone on a post as Deputy Director at MI5. At that time, via the agency of time travel, he met up once again with a familiar version of the Doctor, as well as a much younger Sarah Jane Smith, for whose personal timeline, little time had passed (MA: System Shock). He still worked for MI5 in 1999. (PDA: Millennium Shock). Circa the 2000s, he worked for NATO, having achieved the rank of Commodore. The Brigadier, at this time, asked him for a favour. (BFU: The Wasting)

Sarah Jane Smith recalled in 2010 that Harry had worked in developing vaccines that had saved thousands of lives. (SJA: Death of the Doctor)

Possible fates
One account says that in the early-2000s, to commemorate an anniversary date, Sarah Jane would have meetings with Harry Sullivan. Forat least one, Harry was absent, though Sarah 'talked' to him and raised a toast to him wherever he was. (BFSJS: Buried Secrets)

Another version of event says Harry passed away sometime before 2009. (SJAN: The Wedding of Sarah Jane Smith)

In one timeline, Harry Sullivan was still alive in 2015, and discovered a possible cure for HIV in blood taken from David Daniels. (NA: Damaged Goods)

Alternative timelines
A parallel Earth version of Harry served on a nuclear submarine in a dystopian world ruled by the Silurians. He managed to save the life of the the Doctor's companion, Bernice Summerfield. (NA: Blood Heat)

Another adventure showed Harry turned into a werewolf after being bitten in the 1930s. There are multiple versions of this within the book. (PDA: Wolfsbane)

Personality
Harry was rather old-fashioned and stereotypically English in his attitudes. He often employed slightly archaic language, referring to Sarah affectionately as "old girl" or "old thing". (DW: The Sontaran Experiment) He was depicted as possessing great bravery, one time saving the Fourth Doctor's life from a bomb on Skaro (DW: Genesis of the Daleks) and a "can-do" attitude, adapting well to the many strange situations in which he found himself. He could, however, also be quite clumsy and unsubtle, leading the Doctor to once declare in a moment of frustration, that "Harry Sullivan is an imbecile!" (DW: Revenge of the Cybermen) He had a slightly flirtatious relationship with Sarah. Despite an outward annoyance at that attitude, once when thinking him dead, Sarah admitted she found it endearing (PDA: Wolfsbane).

Relatives
Harry had a brother named Will, whom Sarah Jane Smith met. (BFSJS: Buried Secrets) Will died some time after meeting her. (BFSJS: Fatal Consequences)

Behind the scenes

 * Harry was written in as a companion when it was initially thought that the Fourth Doctor would be played by an older, less physical actor. This would necessitate Harry to handle physical "two fisted" action as the plot required. This was no longer a concern when Tom Baker was cast in the role as he was younger than the previous actors and capable of physical action on his own. Harry was considered redundant and ultimately written out of the show, despite his popularity.
 * Although various spin-off media have depicted Harry living into the 21st century, in real life the actor who portrayed him, Ian Marter, died in 1986. A reference to the character is made in SJA: Invasion of the Bane, when Sarah Jane considers Harry as a possible name for her adopted son, Luke Smith. Perhaps going along with this fact, it has been implied that Harry has indeed already passed away in the present-day continuity of the Doctor Who Universe, as in SJA: Death of the Doctor, when recounting the current endeavors of some of the Doctor's past companions, Sarah Jane refers to him in past tense.
 * After leaving the cast of Doctor Who, Ian Marter went on to write several Target Books novelisations. He also wrote Harry Sullivan's War for them in 1985. In Harry Sullivan's War, the character has become an MI5 operative. Marter was believed to have been planning a sequel at the time of his tragic death from a diabetic coma the following year.
 * Marter also wrote the novelisations Doctor Who and the Ark in Space and Doctor Who and the Sontaran Experiment, putting him in the unique position of adapting two stories in which he himself had acted.
 * There an apocryphal account presented in Doctor Who and the Rebel's Gamble suggesting that at some point Sullivan shared an adventure with the Sixth Doctor.