Head Six: the Number Six only Gaius Baltar can see

Head Six is a woman who looks exactly like Number Six and appears only to Gaius Baltar, talking to him, mocking him and steering his choices from the miniseries to the last episode. Fans called her Head Six because she seemed to live in Baltar's head. Nobody else could see or hear her, so Baltar spent four seasons arguing with thin air in front of crewmates who thought he was cracking up. (Battlestar Galactica (miniseries); Battlestar Galactica, "33")

Who is Head Six?#

Head Six is the face of the Six that Baltar was sleeping with on Caprica. That Six used him to get into the Colonial defence mainframe, and when the Cylons attacked, the network he'd handed over helped them cripple the fleet. Once Baltar got off Caprica and aboard Galactica, a Six started appearing to him, sometimes in a red dress at his side, sometimes in a bright, sunlit version of his house on Caprica. (Battlestar Galactica (miniseries))

She knew things he didn't. In the miniseries she pointed him towards a supposed Cylon device in Galactica's CIC, and he used it to accuse Aaron Doral of being a Cylon. From the first season on she nudged him through crisis after crisis, often by playing on his fear of being exposed as the man who let the Cylons in. (Battlestar Galactica (miniseries); Battlestar Galactica, "33")

What does Head Six want?#

From the start she preaches. Head Six tells Baltar that the Cylons' God has a plan for him, and in "33" she pushes him into repenting before that God, just before the Olympic Carrier is destroyed. The polytheist Colonials and the monotheist Cylons are set against each other through the whole series, and Head Six keeps dragging Baltar across that line. (Battlestar Galactica, "33")

Her other theme is a child. On Kobol she shows Baltar a baby in a crib, the first of a new generation of God's children, and tells him he is to be their protector. That thread runs to Hera Agathon, the first child of a human and a Cylon, and the finale shows protecting Hera was the point all along. (Battlestar Galactica, "Kobol's Last Gleaming, Part II"; Battlestar Galactica, "Daybreak, Part II")

She isn't gentle about it. In "Escape Velocity" she shoves Baltar into a bulkhead and grabs him by the throat, and after he's beaten she hauls him back to his feet. Baltar feels her touch. Nobody watching sees anyone there. (Battlestar Galactica, "Escape Velocity")

Theories the show raises about her#

The series floats several explanations and lets Baltar try each one on.

• She's a hallucination. That's the obvious reading, and it's how the crew see it. Baltar talks to nobody in corridors, labs and meetings, and people stare. (Battlestar Galactica (miniseries); Battlestar Galactica, "33")
• She's a Cylon chip in his head. Baltar wondered if the Six on Caprica had planted something in his brain. A scan aboard Galactica showed nothing physically wrong with him. (Battlestar Galactica, "Home, Part II")
• She's a real Six. In "Six Degrees of Separation" a flesh and blood Six, posing as Shelly Godfrey, boarded Galactica and framed Baltar. Head Six was missing the whole time she was aboard, which hints the two are linked. (Battlestar Galactica, "Six Degrees of Separation")
• She's an angel. Head Six says so herself. After the brain scan comes back clean, she tells Baltar she's an angel of God, sent to protect and guide him, and she says it again aboard the basestar. (Battlestar Galactica, "Home, Part II"; Battlestar Galactica, "Torn")
• Baltar is a Cylon. Once he's living aboard a basestar in season 3, Caprica-Six explains the projections Cylons use to picture their surroundings however they like. Baltar notices his visions work the same way and starts to wonder whether he's a Cylon himself. (Battlestar Galactica, "Torn")

Who is Head Baltar?#

Head Baltar is Head Six's mirror. When the Six who seduced Baltar was killed on Caprica and downloaded into a new body, a Gaius Baltar started appearing to her that nobody else could see. Fans call her Caprica-Six to tell her apart from the other copies. (Battlestar Galactica, "Downloaded")

He pushed her the other way. Where Head Six pulled Baltar towards God and the Cylons, Head Baltar needled Caprica-Six about the billions she'd helped kill, and the guilt helped turn her against the war. He's more cynical than Head Six, and he treats Cylon religion with scorn. (Battlestar Galactica, "Downloaded")

Head Baltar kept turning up after Caprica-Six came to Galactica. In "The Woman King" he's still with her aboard Galactica, unseen by everyone around them. (Battlestar Galactica, "The Woman King")

What the finale reveals about Head Six#

"Daybreak, Part II" settles it. During the assault on the Colony, Baltar and Caprica-Six hear a Six's voice, and they both see Head Six and Head Baltar. For the first time each can see the other's companion. Moments later they come across Hera, who has run off in Galactica's corridors after her rescue, and Caprica-Six closes a hatch with the child in her arms, the exact scene from the opera house vision several characters had been seeing for years. (Battlestar Galactica, "Daybreak, Part II")

In the CIC, with a gun to Hera's head, Baltar tells Cavil that he sees angels in the room. He describes a force beyond them that it doesn't matter whether you call God or gods, and says that God is not on anyone's side. (Battlestar Galactica, "Daybreak, Part II")

The epilogue jumps 150,000 years ahead to a modern city on the second Earth. Head Six and Head Baltar walk through the crowds, long after the real Baltar and Six are dead. They wonder whether the cycle of humans building machines that turn on them will happen again. When Head Six mentions God's plan, Head Baltar says it doesn't like that name. (Battlestar Galactica, "Daybreak, Part II")

So the answer is: neither madness nor a chip. Head Six and Head Baltar are messengers of whatever force guided the human and Cylon survivors to Earth. The show calls them angels and stops there, never saying what they serve beyond that it's real and not on either side. (Battlestar Galactica, "Daybreak, Part II")