Face of Boe

The Face of Boe appeared to be a male Boekind who was little more than a gigantic, humanoid-like head. Although he would rarely do more than grunt vocally, he was able to communicate by means of telepathy.

Early life
The Face of Boe had the distinction of being one of the oldest creatures in the known universe. He was supposedly well known by the 51st century. Jack Harkness commented on the fact that when he was younger on the Boeshane Peninsula, the first person there to join the Time Agency, he was nicknamed the Face of Boe. (DW: Last of the Time Lords)

This has led to the out of universe conclusion that since Jack can not die, he is in fact the Face of Boe.

In 200,000, he was featured on Bad WolfTV, at which time he fell pregnant with Boemina. (DW: The Long Game) A hundred years later, he was the focus of a question on The Weakest Link, in which he was described as the oldest living creature in the Isop galaxy. (DW: Bad Wolf)

Later life
In the year 5.5/Apple/26, the Face of Boe was the sponsor of an event to safely witness the destruction of the Earth by the expansion of the Sun. The event was sabotaged by Lady Cassandra, but the Face was among the survivors. (DW: The End of the World) By 5,000,000,023, the Face was hospitalised in Ward 26 of the hospital run by the Sisters of Plenitude on New Earth. By this time, he was the last of Boekind. Apparently dying of old age, the Face summoned the Tenth Doctor to his ward. When a novice told the Doctor the legend of the Face's last words - a secret which the Face would impart only to one like himself - the Doctor realised that he fit the description of the "wandering traveller".

The Face of Boe eventually recovered, saying that although he had grown tired of the universe, the Doctor had shown him a new way of looking at things. The Doctor asked about the message, but the Face told him that it could wait for their third and final meeting, and teleported away. (DW: New Earth)

Death and aftermath
The Doctor was ultimately reunited with the Face of Boe one last time during his travels with Martha Jones, in the year 5,000,000,053. Boe revealed his last secret with his dying breath after sacrificing himself to save the city of New New York: "You [the Doctor] are not alone". The Doctor dismissed this, as he still firmly believed himself the last Time Lord. This was the only time the Face of Boe communicated vocally with his only words out loud being "you are not alone." (DW: Gridlock)

When the Doctor later met Professor Yana (the Master transformed into a human) the Doctor realised the significance of the name "Yana", which stood for "You Are Not Alone". Yana's memories also indicate that he originated in the Silver Devastation. (DW: Utopia). When Davros asked the Doctor How many have died in your name?; The Face of Boe was among those who appeared in his flashback. (DW: Journey's End)

Personality
The Face of Boe was a mysterious being but because of his near-immortal lifespan he was well known throughout the universe. He rarely spoke with his voice, preferring to communicate through telepathy and when he did use his voice he usually grunted instead of speaking. However he was capable of using actual words as when he told the Doctor that he was not the last Time Lord. (DW: Gridlock)

The Face of Boe was a compassionate and selfless being who was willing to give his life for the greater good. He used up all of his remaining life force to save people who were trapped on the Motorway. Having lived for billions of years, the Face had grown tired of his unbelievably long life but after meeting the Doctor for the second time, he stated that the Doctor had taught him to look at the universe differently. (DW: New Earth)

Behind the scenes

 * In a feature for Doctor Who Magazine, Russell T Davies wrote of a prophecy in which the death of the Face of Boe would be heralded by "the heavens opening". When he wrote Gridlock the following year, he realised after the fact that the Face of Boe's death did indeed cause the heavens to open - his last bit of energy opened the sealed system to free the drivers of New Earth.
 * When the Face met the Doctor for the second time, he stated that he would only meet the Doctor one last time before dying. (DW: New Earth) This indicates that he may possess the ability to see into the future. However it's also possible that he (most likely while he was Jack Harkness, assuming they were indeed one and the same) had heard about his own death in a later time period. Considering Jack was a time traveller and had heard of the Face of Boe (indeed he had been nicknamed the Face of Boe in his youth) this is a more likely explanation for his knowledge about his own death.
 * Despite the revelation in Last of the Time Lords, the actual origin of the Face of Boe is still debated, with Davies himself waffling during the commentary for the episode as to whether Jack really is destined to become the Face of Boe. Producer Julie Gardner, however, has been more assertive in this regard, most recently stating outright at the 2008 San Diego Comic Con that the Face of Boe is Jack Harkness. Similarly, the Torchwood Declassified featurette from Torchwood Series 2 has John Barrowman, David Tennant and Russell T Davies stating that they believe the Face of Boe is Jack Harkness. In-universe, however, we only have Captain Jack's testimony.
 * While promoting Torchwood: Miracle Day, Davies insisted that the idea of Jack living to become the Face of Boe is just a conjecture, and the possibility of Jack not surviving Torchwood remains.