Solidus Snake
Solidus Snake is one of the three "Sons of Big Boss", created as part of the Les Enfants Terribles project. In the project, one of the clones was to carry all of Big Boss' dominant genes (Liquid Snake), one was to carry all of his recessive genes (Solid Snake), and one was to be an exact copy of Big Boss (Solidus)
"Solidus" means "neither Solid nor Liquid" which is fitting seeing as how Solidus is the perfect balance of his two brothers
Background
The early years of Solidus Snake is unknown, the earliest bits we can piece together is that at one point he adopted Raiden and taught him to be a ruthless killer. In that war Raiden was nick-named "Jack the Ripper" for his outstanding ability to kill.
Solidus was also the President at one point in the timeline of MGS (he was the 43rd President), he was known as George Sears while he was in presidency, and acting outside the orders of The Patriots he started the Shadow Moses incident in complete secrecy, telling Revolver Ocelot to persuade Liquid into stealing Metal Gear REX and use it for his own twisted ways. But because Solidus accidentally revealed the existance of Metal Gear REX and the Genome Army he was removed from Presidency and went underground when he heard the Patriots were trying to kill him off. While underground he took ahold of Dead Cell as a way to gain power and destroy The Patriots. Solidus was then killed by Raiden when the two battled atop Federal Hall