Face of Boe

The Face of Boe appeared to be a man 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 Wolf TV, at which time he fell pregnant with Boemina. (DW: The Long Game) 100 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 realized 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 massive 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 shown 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)

Possible origins
After the death of the Master, the immortal Jack Harkness said he lived on the Boeshane Peninsula, where, as the first Time Agent from that place, he was a poster boy and nicknamed the "Face of Boe". Given that the Doctor had just speculated that Jack might mutate in shape rather than age in the traditional way, this implied that Jack might one day age into the being known as the Face of Boe. (DW: Last of the Time Lords)

Although the Face of Boe appeared to have died permanently (DW: Gridlock) this does not disprove the theory that he was once Jack Harkness. The Doctor simply may not have stayed long enough to witness Boe's resurrection, which is possible given that he did not suspect a connection between his two friends at the time and the more life energy Jack expels during a death the longer it takes him to be revived (TW: End of Days). Also, Boe's death may have been a metaphor for a desire to end the current life he was leading. Jack has been shown to have lived several different lives, most of which were presumably ended by his many deaths. (TW: Children of Earth: Day Five) It has also been demonstrated that there are mechanisms by which Jack can be stripped of his immortality. (TW: The New World, End of the Road)

However it is possible that Jack was not truly immortal and instead possessed a tremendous amount of life force, which allowed him to live for billions of years. His countless deaths and resurrections may not have been the result of true immortality; instead they may have been the result of the incredible amount of life force he possessed and every time he died he had to use up some of his life force in order to return to life. At the time of the Face of Boe's death he may have finally used up all his life force and therefore was able to truly die. Another theory is that while Jack's body had permanently died at long last, he might have become a being of pure consciousness, as the Time Lords had intended to do to escape the Time War (DW: The End of Time). Shortly before dying the Face of Boe stated "everything has its time" which presumably means that even Jack Harkness couldn't live forever (DW: Gridlock).

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 realized 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.