Not much is really known about Tyrol's past, only that his father was a priest and that his mother was an oracle, or a spiritual advisor. Galen himself never really embraced religion, having thought it was a rather ridiculous practice. In fact, when he was alone, he would often times he would sneak into his mother's prayer room and dance around unclothed, with porn magazines just to defy the gods that his mother and father served so passionately (The Eye Of Jupiter) .
He's worked with Commander Adama five years previous to the series staring up, as is said by the commander in defence of the Chief when he is accused of helping the Cylons sneak an agent aboard the ship (Litmus) . He started working as a deckhand at age eighteen. The ships he has worked on in the past include the Columbia, Atlantia, and the Pegasus (Resistance) . Galen is the highest ranking and one of the most well respected and admired NCO's in the entire fleet. He is , however, like his wife Cally, prone to temper and can over react to tragedies to easily. This is first displayed when part of the Galactica had to be vented in order to put out a couple of dangerous fires. They lost 86 people, and Galen was furious with the XO Tigh for his decision.
When the series starts up, Galen has an affair with Lt. Sharon 'Boomer' Valerii, despite the fact that it is against the Military Code of Conduct. While it is obvious that he does ,in fact, love her, their relationship meets rocky grounds when Boomer begins to think she might be a Cylon (Water) , and Galen is unsure if she isn't. Their relationship comes to a crashing halt during an investigation on how the Cylon Dural got on board and blew himself up. Sergeant Hadrian, who is also the Master of Arms aboard Galactica launches the investigation with permission from the Commander, and she questions Tyrol and his deck crew as to the whereabouts of the Chief during the explosion. After many interrogations, Galen's deckhand Socinus is brought back to the wardroom, and questioned again. After neither Galen nor Boomer confess to the crime, Socinus is arrested, stripped of his rank and sent to the brig. Racked with guilt over landing one of his own 'kids' in the brig, Galen goes to Boomer and tells him their relationship is over, and that " we are NOT worth that." (Litmus).
When the colonies find a planet that is believed to be the legend planet of Kobol, Galen has his deckhand Socinus released from the brig, but scolds him for what he did. Afterwards, him, Socinus, Cally, Seelix, Baltar, Karma, Tarn, and Crashdown go to the planet , but while they are in orbit, Karma is killed when his raptor is shot down (Raptor Part I ). Tarn (Scattered) and Socinus (Valley of Darkness) die next do to an injury and an o.d. . The Chief is struck with a feeling of grief over the loss, and becomes weary of the situation. Despite his and Baltar's advice , Crashdown comes up with a potentially suicidal mission to stop the Cylon's missile that they built to shoot at the SAR rescue team. When Cally refuses to go on the mission, frightened, and nerves rattled, Crashdown pulls a gun on her. Being protective of his crew, especially Cally, Galen pulls a gun on Crashdown and tells him to lower his gun. However, when it looks like Crashdown is seriously going to kill her, Baltar shoots him in the back. The loud gunshot gives off their location and the crew is chased through the forest. Galen is shot in the arm, but after he gets the right angle and tells Baltar to cover him, he shoots a grenade at the missile and destroys it, allowing the SAR team to land safely . When Lee asks about Crashdown's death, Galen and Baltar lie and tell him that Crashdown died defending everyone from the Cylons.
Upon return to Galactica Galen is arrested and questioned by Tigh due to his old relationship with the Commander's would-be assassin Boomer. While in prison, he agrees to help Baltar get the truth out of Boomer, and Baltar injects him with a drug that causes his systems to shut down. After Baltar tells Boomer that the Chief is going to die, she tells him everything. He later injects the antidote into Galen and he is released. After he is released, he goes with the security to Boomer's testing cell, and Cally fallowed. Upon arriving at the cell, Cally shoots and kills Boomer, and as Boomer dies she tells Galen that she loves him. This makes him even more distant to his crew then he already is. And although Cally shot someone he loved dearly, he vouched for her which led to her getting a 30 day sentence in the brig instead of something worse.
Helo's return to Galactica shook Galen a bit when a very pregnant Sharon was with Karl, carrying his ( Helo's ) child. This stress, along with maintaining the vipers with a disgruntled crew, gets to him. When he finds Helo in Sharon's old viper ,drinking, the two start to talk. However, Helo is insulted when Galen calls his unborn child 'that freak in her belly' and the two have a small fist fight. Galen stops when he realizes he is overreacting. He soon becomes bent on making a new Viper from scratch. His crew, including Cally, think he's a little mad, and he tells them not to help. During the time he makes a very strong alcoholic drink to trade on the black market for parts. He is surprised when, after being caught by Tigh, Tigh says he knows someone who can get him the parts. Starbuck, Dee, Cally, and many others join the Chief later on and help him complete the viper. Helo helps him come up with a material to cover it. It is called The Blackbird, and has the name 'Laura' painted on both sides of the nose, after Mme. President Laura Roslin, who is honoured by the gesture (Flight of the Phoenix).
Despite his dislike for Cylons, while aboard the battleship Pegasus he and Lt. Agathon stop a crew member from raping Helo's Sharon. Galen kills the attacker in the process and he and Helo are arrested by Commander Cain. Adama comes to visit them, telling them of his plan to rescue them, and scolds them for their stupidity. The two of them are beaten by two crew members, but saved by EO Jack Fisk. When the humanoid Cylon Gina escapes and kills Cain, the two are released and returned to Galactica .
After awhile, Galen starts having nightmares where he goes up to the balcony of the Hanger Deck and jumps from the railing to his death. During one of his nightmares, Cally is looking for him, and when she finds him he beats her in a nightmarish rage, unaware of what he is doing. When he wakes up and sees Cally, beaten, bloody, and looking to be within an inch of her life, he takes her to Sickbay. Racked with guilt and confused with his dreams, he requests religious counciling and is assigned to Brother Cavil ( who is later revealed to be a Cylon) . Cavil explains that the dreams might be Galen's subconscious asking him if there is a possibility he is a Cylon ,and he wants to die because of it. Galen wonders about what he said ( Lay Down Your Burdens Part I).
After awhile, he returns to Sickbay to apologize to Cally. Her jaw is wired to heal , but she tells him that she believes it wasn't his fault, and tells him she cares about him. When the colonies find a new planet, and Baltar, newly elected president of the twelve colonies, says they shall settle there. 380 days into the occupation, Galen has become the President of the Workers Union, and is married to Cally, who is very pregnant with his child. Soon after, the Cylons find the planet , and an occupation begins. During the occupation, his son, Nicholas is born and he is dedicated to the service of the gods Ares and Apollo, on Cally's request ( The Resistance : Part X ) . By the 67th day of the occupation, Galen teams up with Tigh to form a resistance army to overthrow the Cylons. By the 134th day the Resistance Army is in full bounty and Galen has teamed up with Samuel Anders to get their planet back. When Cally is captured to be one of the people to be executed, Galen rushes to rescue her. After she is safely hid, he goes back to the Resistance. After their rescue, he becomes a member of The Circle, but it soon disbands after Gaeta is accused. During a 'dance' that the Commander puts on, Galen fights the Commander and wins, but the Commander has the last word when he says he let everyone get to close.
When the fleet runs out of food and a planet rich with algae is found on Athena's dradis during a search, Galen, Cally, and a few crew members and civilians go down to the planet to collect the algae. During the 14th day of the supply mission, Galen is drawn towards the mountains of the planet, and discovers the lost Temple of the Five/ The Eye Of Jupiter, which is said, like the Tomb of Athena, to be another place to find a way to Earth. While he and other crew members plus Cally are in the temple, Galen takes on a 'softer look, you even talk softer' demeanour, as said by Cally. Despite his arguments, the Commander gets him people off the planet after threatening the Cylons that he will indeed nuke it ( Eye of Jupiter & Rapture) .
During a rough patch in his marriage with Cally, the two of them get trapped in an airlock while trying to repair some of the gearing systems. As they are working to repair the leak that caused the doors to shut, they argue more, Cally saying he's trying to pretend that everything is the same as before New Caprica when it's not. She says they don't spend enough time with their son, which is true. As the situation worsens, Galen admits to Cally that he was being selfish, and all he wanted was to have her with him again like old times, and he didn't even think of Nicky. He apologizes and tells her he loves her. Commander Adama tells them that the only way to get them out is to blow the hatch and have them thrown into a waiting raptor as the air flies out. Despite the danger it is their only option. The Tyrol's prepair for their exit timidly. When they are safely rescued, they are taken to sick bay. Seelix brings Nicky to them from his daycare, and Galen takes Nicky over to the hyperbaric chamber, and tells her that he will make sure their marriage works and that they will be okay, and spend more time with their son ( A Day In A Life).
After the people at the fuel refinery are found to be in horrible conditions, Galen buts heads with the Commander over what is to be done. He demands better living conditions and better pay for the workers, and says that it is horrendous that there are children working in the refinery. This launches a revolt amongst the hard labour workers, including his deckhands and his wife. It all ends when Commander Adama calls to have Cally arrested and shot. Horrifide and obviously not wanting to lose his wife, he backs down, and tells Cally to call it off. (Dirty Hands) .
Recently a strange music that could only be heard by him, Sam, Tigh, and the president's assistant Tory Foster, haunts Galen. When it leads the four of them to a room in Galactica the realization dawns on them...they're Cylons. Four of the final Five models. After a small argument, an alarm goes off, signalling a Cylon attack. They all rush to their rightful places and work like nothing ever happened (Crossroads Part II).
Galen and the other three final Cylons continue to try and figure out what they were going to do in S4. When Starbuck returns to Galactica, Galen finds evidence to doubt that Kara is a Cylon, pointing out that the viper Kara came back in is new. He continues to ponder the meaning of Kara's new viper, and becomes fascinated by it. He spends time with his fellow Cylons , having to wait until Cally falls asleep, which takes awhile, as she had insomnia problems lately. He tries to console a worried Tory, making Cally suspect he's having an affair, when he isn't. Tory then tells him at the end of the epsiode that Call commited suicide. (The Ties That Bind ) After Cally's death, which Tory claims was a suicide, Galen seems to slip into an emotional coma. He becomes distracted while working, and almost causes Racetrack's raptor to crash. She tells him she understands he's in pain, and says no one died in her emergency landing, so there is no harm done. He has an illusion in Joe's Bar after Papa Adama sits next to him. In his illusion, he thinks that Adama figures Cally commited suicide because she found out Galen was a cylon, and that her child was a hybrid. This starts a string of occurences in a few minutes, and Galen has a complete break down, yelling about how Cally wasn't an angel and he isn't going to make her out to be one after her death. It's obvious he's still in grievence . He tells Adama that he should just demote him, have him transfered if he's become such a big problem. As he continues to yell, creating a scene in the bar, Adama does demote him , and tells him he will be transferred to another ship. (Escape Velocity)
Galen becomes bent on figuring out what happened to Cally, after he gets a feeling that she didn't commit suicide after all. He goes to the airlock where she supposedly commited suicide. Tory still tries to get him to not see that Cally commited suicide. He still works on the ship he is transferred too, and gets drawn into Baltar's new religous cult. He overhears one of Baltar's cermons, and walks to one of his cult gatherings. Baltar sees him and tells him to come closer. When Baltar tells Galen he thinks that him finding religon would be what Cally would have wanted, Galen snaps. He yells at Baltar, telling him that he didn't know Cally at all, and had no right to tell him what Cally would have wanted. Later in the episode, he thinks about his life, and reconciles with Baltar, after Baltar tells him how sorry he is for his loss, and he's right, he didn't know Cally, but he does know she would have wanted him to go on with his life. (The Road Less Traveled)