"You're pretty good!"
Child of The Boss and the Sorrow, Ocelot grew up away from his parents, under the influence of the Patriots from a very young age. When he eventually reconnected with his birthmother, neither of them was aware of it, and Ocelot's role as double agent for the KGB and CIA in investigating Colonel Volgin's GRU faction took center stage. As Naked Snake embarked on Operation Snake Eater, Ocelot began to investigate the Philosopher's Legacy, taking a liking to Naked Snake and forging a rivalry in the process.
Years later, Ocelot and Naked Snake (now known as Big Boss) joined a faction of the Patriots under the influence of Major Zero. The Les Enfants Terribles Project led to the birth of Solid and Liquid Snake, with Big Boss leaving shortly thereafter to organize his own faction. Ocelot began to resent the policies of the Patriots, but kept his thoughts to himself.
Ocelot incited Liquid Snake's insurrection at Shadow Moses. He tortured and killed DARPA Chief Anderson. He challenged Solid Snake to a duel, swearing to kill Snake after losing his hand to Grey Fox's blade. He also tortured Solid Snake in the same fashion. After all was said and done at Shadow Moses, he took the REX data for the grand scheme of the Patriots.
Ocelot joined up with Sergei Gurlokvich with plans to steal the Metal Gear RAY being developed by the US Navy, then took it for his own after double-crossing Sergei. Everything that occurred in the years that followed - the construction of Big Shell, the production of more RAY units, the creation of Arsenal Gear, and the rise of the Sons of Liberty - were aided by Ocelot's actions. He provided the tech that Fortune used to deflect bullets, and revealed this to Solidus Snake as he bid them farewell, taking off in a Metal Gear RAY unit.
With Liquid Snake's personality merged with his own, Ocelot (know more accurately known as Liquid Ocelot) led his own insurrection with Private Military Corporations shaping the face of international affairs, and the concept of power. Ocelot faced down Solid Snake in the RAY, while Snake piloted the REX, which resulted in both being rendered useless. With his own plans to unravel the Patriots complete, Ocelot challenged Snake to one final duel, his last words being the same that he uttered to Big Boss during Operation Snake Eater.
While Revolver Ocelot was actually working against the Patriots for the majority of his involvement with them, Ocelot leads both Solid Snake and players on to believe he is the enemy throughout the entire series. Though his aims may have been justified all along, Ocelot thieved, manipulated, and killed countless people to achieve that end. He knew this major goal would require some drastic actions, and was prepared to travel down the darkest of paths to see to its success.
I chose his theme from Metal Gear Solid 4's final boss fight because it represents every one of Ocelot's appearances in the previous games.