(The reimagined series; Caprica and Blood & Chrome)
Galen Tyrol was a Cylon descended from the Thirteenth Tribe. Escaping a Cylon uprising on Earth, Tyrol was one of five who tried to prevent - and later end - a similar war occuring amongst the human Twelve Colonies. Faced with another uprising, he was exiled to live among humanity and given false memories. It is unknown if "Galen Tyrol" is his real name, or part of this cover.
Tyrol was born on the planet Earth at approximately the same time the Twelve Tribes abandoned their homeworld of Kobol. Guided by the Messengers, he joined a group of five scientists who had advanced warning of an impending war on Kobol between its biological settlers and their own mechanical labour force, which was to destroy the planet in a slave uprising. He was killed in a nuclear blast while at a market, and his consciousness downloaded into an artificially grown copy stored on a ship in orbit. Not posessing FTL technology - or at least not on the ship - the five spent centuries travelling in search of Kobol's people, but found them some 2000 years later engaged in a devestating war with their own mechanical Cylon labourers. Tyrol and the other four were able to enter negotiations with these Cylons, and they called an end to hostilities with the humans after twelve years of fighting in exchange for technology on organic memory transfer.
Over the next several years, the Cylon race went through considerable change, with biological Cylons being created with human form and emotions. The first of these prototypes, a Number One, directly assisted in the creation of the others, but eventually lost faith in his creators. A revolution took place, in which the Final Five were killed, and the mechanical Cylons were deprived of self-awareness, creating a new society in which the eight humanoid models and their many copies were effectively slave-masters. Tyrol, as punishment for his love of humanity, was reactivated many years later and sent out to live in the Cyrannus star system, given fake memories about having been the son of religious elders.
Not long after arriving in human society, Tyrol enlisted in the Colonial Fleet, and demonstrated mechanical skills on several battlestars such as Pegasus before being assigned to Galactica as its deck chief ahead of its decommissioning. While there, he developed romantic feelings for a Cylon Sleeper Agent, a Number Eight calling herself Lt. Sharon Valerii. The relationship was taboo due to the conflict-of-interest between a pilot and the deck chief, and despite their efforts to keep it hidden it became an open secret known to much of the ship, but tolerated by Galactica's senior staff due to the decomissioning. The two had plans to transfer together to Pegasus, or to otherwise retire entirely and start a family.