"Brother!!"- Liquid Snake
Liquid Snake is the main villain of Metal Gear Solid and, seemingly, its sequels Sons of Liberty and Guns of the Patriots.
Liquid Snake, along with Solid Snake and Solidus Snake, was created in a lab and cloned from Big Boss, intended to be a military super-weapon. He held a powerful grudge against Solid Snake, believing that he had inherited the superior genes. This, however, was false, as it was actually Liquid Snake who was the strongest of the three. Liquid's primary goal was to defeat Snake in combat and prove his superiority.
In Metal Gear Solid, Liquid was the leader of the FOXHOUND terrorist unit operating on Shadow Moses Island trying to use the Metal Gear REX. He was known for his extreme perseverance; not only does he survive being exploded in the helicopter he uses the first time you fight him, but also manages to stay alive after being shot repeatedly in the cockpit of the Metal Gear REX during the final battle and then being beaten up by Snake and falling from the top of the robot, and returns once again in the final escape sequence where he is hit by machine gun fire, surviving that as well... only to abruptly die from the "Foxdie" genetic disease right afterwards.
In the sequel, however, it is revealed that Liquid's spirit is still possessing Revolver Ocelot, who had Liquid's hand attached to him, and is in complete control of Ocelot during the fourth game, where Liquid is once again presented as the main antagonist. Given the fact that he was already so hard to kill before and the presence of other light supernatural elements in the series such as Psycho Mantis and the Sorrow from MGS3, this was not that hard to believe, but in the ending of MGS4 it was finally revealed that Liquid really was dead since the first game after all, and Ocelot had actually hypnotized himself into believing he was possessed by Liquid.