|“||I was trying to take down a system dedicated to the worship of false gods. I became a false god all over again.||”|
|— Gabriel to Octavia [src]|
He was an original member of Mission Team Alpha and the former thirteenth Prime. After a moral awakening, Gabriel defected from the Primes and formed the Children of Gabriel to take down the Primes. His current host is Xavier.
Gabriel was born on Earth and eventually joined the Eligius Corporation. Before the first Nuclear Apocalypse, Gabriel was selected to join Mission Team Alpha to explore life and human survival on Alpha.
Shortly after arriving on Alpha, Gabriel's girlfriend, Josie, was killed by her father, Russell Lightbourne, who was under the influence of the eclipsed-induced psychosis. Following Josie's death, Gabriel started working with Russell to figure out how to bring her back using their Mind Drives which already stored her consciousness. He incubated some of the Nightblood embryos they brought from Earth so that he can upload the Mind Drives on the children, but the children always ended up dying because their minds weren't developed enough to hold an entire consciousness. After 25 years of experiments, Gabriel eventually figured out that they needed to use a fully developed brain and erase the host's mind completely in order to upload the entire consciousness from the Mind Drive. This worked successfully and brought Josie back.
By the time that Josephine was brought back, Gabriel was dying of cancer. Unwilling to lose him, Josephine resurrected Gabriel through his Mind Drive which he hadn't wanted to happen. After his first resurrection, the two shared a dance and Josephine promised to love Gabriel forever.
Following Gabriel's breakthrough, the Primes started using the Mind Drives to bring themselves back by uploading their consciousness into Nightblood hosts. The Primes at some point started experimenting with the Red Sun toxin with Simone trying to weaponize it while Gabriel focused on creating an anti-toxin. Gabriel discovered a way to use the toxin to create a waking dream as part of "a happy accident." At some point, he and Russell experimented using Gabriel's variant of the toxin to see if anything of their hosts survived. The experiment showed that nothing of the host survived, but gave them a revelation about their purposes: Russell's was to make them into gods while Gabriel's was to stop it from happening.
Eventually, in 2211, Gabriel started feeling guilty and started to rebel against the Primes. To stop the resurrections, Gabriel destroyed the remaining embryos which had been genetically engineered with Nightblood. Simone Lightbourne intended to burn Gabriel at the stake for his actions, but he was helped to escape from Sanctum by Ryker Desai who left his cell door open. He formed the Children of Gabriel group to help take down the Primes.
Gabriel trained an null named Eduardo, who was left in the Offering Grove as an infant. He taught him how to remove the Mind Drive so that he won't be brought back. However, as Gabriel was about to die of age, an infant Nightblood named Xavier was born. Eduardo saw this as a sign to bring Gabriel back. Gabriel became close with Xavier who was also like a son to him and rejected Eduardo's idea that they use Xavier to resurrect him and believed that he had talked Eduardo out of it. After Gabriel died, Eduardo went against his wishes and used Xavier as a host to resurrect Gabriel. Gabriel was so upset at Eduardo for bringing him back that he killed him out of anger. He was left with no one to remove the Mind Drive.
Since no one else knew about it, Gabriel decided to pose as Xavier among the Children of Gabriel. They assumed that either the Old Man Gabriel had died or abandoned them. Gabriel posed as Xavier for ten years while the Children of Gabriel believed that Eduardo had been killed by the Sanctum Guards around the same time that Gabriel disappeared.
Gabriel was also extremely interested in the temporal anomaly on Sanctum. It's implied that he visited it often to see hallucinations of Josephine who he never stopped loving. According to a comment Gabriel made, he spent at least 150 years studying it by the time the people from Earth arrived though Gabriel apparently couldn't uncover anything about the anomaly and how it worked.
Throughout the Series
Gabriel seems to be a hardworking scientist. He also had caring feelings for Josephine, but their relationship ended when he disagreed with the Primes' own ways and defected from Sanctum. He implies that Josephine is both his darkest fear and greatest desire.
By the time his last host was dying over old age, Gabriel was ready to die for good, but adopted a boy he rescued named Eduardo who he taught to remove the Mind Drive. Though Eduardo wanted to bring him back again through a new free-born host, Gabriel was ready for his life to end and enraged to the point of killing Eduardo for resurrecting him again when he was brought back against his will. It's shown that he never wanted to be resurrected the first time either but Josephine couldn't bear to lose him.
After his latest resurrection, Gabriel became ashamed of what he had become to the point that he chose to hide amongst his followers as his host Xavier. Gabriel states that he dedicated himself to bringing down a system of worship of false gods, but instead became a false god all over again.
He was tall and slender with brown eyes, brown hair and light tan skin.
He was in a romantic relationship with Josephine Lightbourne, but their relationship ended when he disagreed with the Primes' own ways and defected from Sanctum. He implies that Josephine is both his darkest fear and his greatest desire. A hallucination of Josephine suggests that Gabriel keeps going to the anomaly to see her which he doesn't deny beyond the last timepp as he had gonppe to help Diyoza and Octavia. When reunited with Josephine, Gabriel was happy to see her, but tearfully refused the idea of saving her so they could be together. He was visibly upset after her EEG flatlined.
At some point, after abandoning Sanctum, Gabriel developed a father-son bond with a boy named Eduardo he'd rescued. Though Gabriel wished to die with his last host, Eduardo loved him too much to let Gabriel go, though Gabriel thought he'd talked Eduardo out of his plan to resurrect Gabriel through Xavier by the time he passed away. In his anger at his resurrection, Gabriel killed Eduardo, something he showed remorse for while explaining the story to Diyoza and Octavia. When confronted by Xavier's sister Layla, Gabriel stated that he also loved Xavier like a son.
|Red Sun Rising||Flashback|
|The Children of Gabriel||Appears|
|The Face Behind the Glass||Appears|
|The Gospel of Josephine||Appears|
|The Old Man and the Anomaly||Appears|
|What You Take With You||Appears|
|Ashes to Ashes||Appears|
|The Blood of Sanctum||Absent|
- Josephine Lightbourne: "What do you say, Gabriel? You ready for phase two?"
- Gabriel Santiago: "That's what the embryos are for."
- -- Red Sun Rising
- Gabriel Santiago: "The key is a fully developed brain. Once the adult host's consciousness is gone, the mind stored in the drive uploads with ease. Now, we still have loads of tests to run, but so far, Josie's consciousness, herself, appears to be whole. In other words, Eureka. More soon."
- -- The Gospel of Josephine
- Tosh: "You're gonna shoot me now?"
- Gabriel: "I'd really rather not. The rest of our unit is expecting you."
- Gabriel (to Octavia and Diyoza): "The harder you fight - the faster you die."
- -- The Gospel of Josephine.
- Gabriel (to Octavia and Diyoza): "I can wait longer than you can."
- -- The Gospel of Josephine.
- Gabriel (to Diyoza): "Did Russell tell you your child will be considered worthless? You don't have the blood, and neither will she. They'll call her a null. She won't be able to bear children. She'll clean toilets or volunteer to die as a guard or attend to the dead."
- -- Memento Mori.
- Gabriel (to Octavia about her hand): "It's not just a tremor. It's a message. The anomaly's calling you... just like it called me."
- -- Memento Mori.
- Brooke (experiment while trying to work the Mind Drives)
- Eduardo (killed in anger over his resurrection)
- Clarke Griffin (heart temporarily stopped; revived)
Notes and Trivia
- He is considered to be a demon in Sanctum. Gabriel himself implies that he is considered one not only for his rebellion, but for destroying the embryos in particular.
- He and Russell jail-broke the Mind Drive.
- In the script, Russell also mentions that Gabriel created the serum used on the hosts before they implant the Mind Drive and that it took him over twenty years.
- He founded the Children of Gabriel.
- He wrote a book chronicling the effects of the Red Sun called "The Double Helix Under the Double Suns." 
- Back in 2275, Josephine Lightbourne mentioned that his last host must be at 95 years old.
- That host is confirmed dead and he currently has a new host that he has had for ten years. This means that his host was in his 80s when he died.
- He is the only Prime that came alone. The others had a family member(s) come with them.
- His original body died of cancer. His previous host died of old age.
- Unlike the other Primes, it is unknown how many hosts Gabriel has had. Given the old age of his previous host, he likely hasn't gone through as many hosts as the other Primes who have each had upwards of six.
- In "Adjustment Protocol," Russell states that Gabriel is in his fifth or sixth host. However, Russell was unsure of the exact number and Gabriel didn't offer it to him.
- At the end of "Matryoshka," Gabriel spoke in Spanish saying "death is life".
- Both of Gabriel's known resurrections were because someone he loved couldn't let him go.