Who Are the Final Five Cylons in Battlestar Galactica?

Spoiler note: this page names the show's biggest late reveals, so stop here if you haven't finished Battlestar Galactica.

The Final Five are Saul Tigh, Ellen Tigh, Galen Tyrol, Samuel Anders and Tory Foster, five humanoid Cylons who spent most of the series believing they were human. Four of them learn the truth at the end of season 3, and the fifth, Ellen, is revealed during season 4. (Battlestar Galactica, "Crossroads, Part II"; "Revelations"; "Sometimes a Great Notion")

For the full background on the Five, see the main Final Five page. This one answers the questions people actually search.

Who is the fifth Cylon?#

It's Ellen Tigh. Saul had killed her as a Cylon collaborator, and the Colonials, along with the Number Three who recognised the Five, took her for dead. In fact she had downloaded into a new body on a Resurrection Ship and was Cavil's prisoner. On the ruined Earth, Saul touches a post box hatch and sees a memory of her trapped under rubble in the holocaust, telling him everything is in place and they will be reborn together. That's how he knows. (Battlestar Galactica, "Sometimes a Great Notion"; "No Exit")

Why are they called the Final Five?#

Baltar coined the name. In "Torn" he notices he has only seen seven models on the basestar and asks about the other five, which Caprica-Six says the Cylons don't discuss. They also turned out to be the last Cylons revealed. They aren't the newest models. They are older than the seven models the other Cylons know, and they helped create those models. They're also called simply "the Five." (Battlestar Galactica, "Torn"; Battlestar Wiki, Final Five)

That's where the old count of twelve humanoid models comes from: the seven models the other Cylons knew, plus the Five. "No Exit" adds a thirteenth, Daniel, the Number Seven, whose line Cavil wiped out before the Five were sent to the Colonies. So twelve is the count the show used for most of its run, and thirteen is the total once Daniel is included. (Battlestar Galactica, "No Exit"; Battlestar Wiki, Humanoid Cylon)

How were the first four revealed?#

Tigh, Tyrol, Anders and Foster each start hearing the same strange melody as the fleet approaches the Ionian Nebula. When they meet in Galactica's gym, they understand. Co-executive producer Mark Verheiden said in a 2008 interview that they only realised what they were on reaching that region of space. They decided to stay loyal to the Colonial Fleet, since they had believed themselves human for as long as they could remember. (Battlestar Galactica, "Crossroads, Part II"; Battlestar Wiki, Final Five)

An earlier trail leads to all five. A Number Three dies and downloads repeatedly to glimpse five robed figures, and at the Temple of Five on the algae planet she sees their faces and says "You were right" to Baltar before she dies. Cavil then has her whole line boxed. When she's later unboxed she knows the four in the fleet, but like everyone else she believes Ellen is dead. (Battlestar Galactica, "Hero"; "The Eye of Jupiter"; "Rapture"; "Revelations")

Why were they hidden?#

Because Cavil hid them. According to the account Anders gives once his Cylon memories return, the Five worked on Earth and rebuilt resurrection technology, then travelled to the Colonies and made a deal with the Centurions. They built the humanoid models, Cavil first, and he turned on them for giving his siblings human weaknesses. He boxed them, then released them into new bodies with false memories and put them into human society. Saul went first. (Battlestar Galactica, "No Exit")

The wiki also says Cavil erased knowledge of the Five from his siblings, and that his was the only model that kept it, though it adds that the Fours knew about Ellen in No Exit. Caprica-Six tells Roslin she is programmed not to think of them. (Battlestar Galactica, "No Exit"; Battlestar Wiki, Final Five)

What happened to the Final Five?#

Anders was shot in the head during the mutiny, regained his Cylon memories, then fell into a coma. The rebel Cylons put him in a Hybrid tank, and from then on he ran Galactica's systems as a Hybrid. (Battlestar Galactica, "No Exit"; Battlestar Wiki, Samuel Anders)

The finale, "Daybreak", settles the rest. To end the standoff over Hera, the Five offer Cavil resurrection and link up through Anders's tank to send it, and in doing so they see each other's memories. Tyrol sees that Tory killed Cally, breaks the link and strangles her, snapping her neck. With the Resurrection Hub gone, her death is permanent, and since each of the Five held part of the secret, there's little chance of rebuilding resurrection without her. (Battlestar Galactica, "Daybreak, Part II")

Once everyone has left the ships on the new planet, which the survivors name Earth, Anders flies Galactica and the rest of the empty fleet into the Sun, and dies with them. Kara Thrace says goodbye to him first. Tyrol tells the Tighs he's going to live alone on a cold island in the northern highlands, and Saul tells him he forgives him for killing Tory. Saul and Ellen, reconciled, walk off together to spend the rest of their lives on the new Earth. (Battlestar Galactica, "Daybreak, Part II"; Battlestar Wiki, Saul Tigh, Ellen Tigh, Galen Tyrol)

Canon notes#

The "Rapture" podcast said the Threes were the first entire model to be boxed, which implied the Five never were. No Exit contradicts that, since the Five were temporarily boxed before being sent to the Colonies, and it also establishes that Cavil destroyed the Number Seven line. (Battlestar Wiki, Final Five; "No Exit")

The extended cut of "Razor" has the first Hybrid predict that four would awaken and "the fifth, still in shadow" would later come toward the light. That fits Ellen's late reveal neatly. (Battlestar Galactica, "Razor", extended version)

Interpretation: my read is that keeping Ellen out of the group scene was the show's best structural choice. Four people finding each other in a gym is a shock. Saul alone in the water on Earth, realising it's his wife, is the shock with a cost attached.

Where to go next#

For where the Five came from, read the Thirteenth Tribe. For the Cylon who hid them, read Number One, called Cavil.