From the Metal Gear Wiki

Liquid Ocelot
Liquid Ocelot, circa 2014.
Also known as	Revolver Ocelot Liquid Snake
Nationality	American British
Affiliations	The Patriots Outer Heaven
Game series	Metal Gear Solid
Appears in	Metal Gear Solid 2: Sons of Liberty Metal Gear Solid 4: Guns of the Patriots
First game	Metal Gear Solid 2: Sons of Liberty
Created by	Hideo Kojima
English voice actor	Cam Clarke (MGS2) Patric Zimmerman (MGS4)
Japanese voice actor	Banjō Ginga (Liquid Ocelot)

Liquid Ocelot was the result of the merger of two beings: the spirit of Liquid Snake and the body of Revolver Ocelot via an arm transplant conducted in Lyon, France, after Ocelot's arm was cut off by Gray Fox during the events of Shadow Moses.

Liquid first emerged when Ocelot was using Sergei Gurlukovich and his mercenary army to take over the U.S.S. Discovery and steal Metal Gear RAY for the Patriots. After Ocelot murdered Gurlukovich and Marine Commandant Scott Dolph, and was getting ready to steal RAY, Solid Snake emerged from his hiding place and confronted Ocelot. Liquid took over Ocelot and made his presence known to Solid, telling him he was "drowning in time".

[edit] The Manhattan Incident

Two years later, Liquid planned an opportunity to gain power. Leaking information to Philanthropy regarding Arsenal Gear, Liquid gained full power over Ocelot during the climax of the Manhattan Incident, and seized the original model of Metal Gear RAY. Liquid then fled, using Ocelot's knowledge as a Patriot spy to find them and launch a revolution against them. However, Solid Snake theorized that Liquid was acting on false information and never found them. In addition, before Liquid took off in RAY, he said "Why do you think I chose Ocelot as my host?" This would indicate that Liquid somehow arranged for his arm to be attached to Ocelot. Liquid's personality would often take over Ocelot's mind whenever Solid Snake was in close proximity.

From then on, Liquid set up his final steps to plunge the world into total war. By the year 2014, wars relied heavily on armies of mercenaries to do their work, and Liquid capitalized on this. Eventually, the five largest PMCs were run by a "mother company" which Liquid had set up, known as "Outer Heaven". Sometime after the Big Shell Incident, Liquid Ocelot began a partnership with ex-Dead Cell member, Vamp. It is unknown why Vamp is aligning himself with Liquid at this time, and given his current uniform, it is speculated that Vamp may even be the commander of Liquid's personal army, the FROGs. Liquid also kidnapped Naomi Hunter to help him create a third generation version of FOXDIE which directly affects the SOP system many soldiers currently use. At one point, as Solid Snake draws near to attempt to eliminate Liquid, he activated what was presumably the new FOXDIE virus, and every soldier on the battlefield in front of him besides Johnny Sasaki was affected by it. It's unknown why Johnny is immune to FOXDIE at this point.

[edit] Ocelot's Status as a Spirit Medium

Revolver Ocelot's parents are The Boss and The Sorrow, two legendary war heroes and members of the famed Cobra Unit. The Sorrow was a gifted soldier with the power to speak with and summon fallen soldiers, to better learn strategies on how to approach enemies on the battlefield. This power as a medium was evidently passed onto Ocelot, unknowingly giving him the power to speak with the dead himself. It can be presumed that during the Shadow Moses Incident, Liquid Snake must have promised Ocelot that if for some reason he didn't survive, Ocelot could graft his own arm onto Ocelot's stump, giving him full usage of both arms once again. Obviously, Liquid was privy to the fact that Ocelot had powers as a spirit medium, even though Ocelot himself may not have been aware of this fact.