Solid Snake's biological father. Commonly portrayed as a very old man in a suit underneath a khaki trenchcoat, and wearing an eyepatch on his right eye.
Big Boss is deceased really. However, the US government has managed to preserve his incinerated remains, and with this, they can study his genetic code. And by studying his genetic code, they can find out what genes caused Big Boss to be "the greatest soldier of the 20th century"; these genes are called the "soldier genes".
Big Boss's genetic legacies were going to be octuplets, but 6 were intentionally aborted so that Solid Snake and Liquid Snake would grow to be incredibly strong. However, Solidus was the survivor of the abortion, and is thus the third.

Big Boss fought Solid Snake twice, once in the first Metal Gear game (set in Outer Heaven), and the second Metal Gear game (set in Zanzibar Land).
It is highly possible that Solid Snake is Big Boss's greatest genetic legacy, despite their hatred before BB's death. After all, who else could take out an almost-immortal soldier (Liquid Snake that is)?!

It has been suggested that Big Boss is the main character for Metal Gear Solid 3 : Snake Eater. We won't know until the game is released in November of 2004....

The Greatest Soldier of the 20th century. The father of Solid, Liquid and Solidus Snake, resulting from the Les Enfants Terribles (The terrible children) project. Originally known as Naked Snake, he earned the name Big Boss after defeating his mentor, the Boss, during Operation Snake Eater. Lost his right eye during said operation.

Founded FOXHOUND in the early 1970s, he subsequently betrayed the United States, probably due to their betrayal of his mentor. Founded Outer Heaven and built Metal Gear, a nuclear-equipped walking battle tank, based on an earlier version which he himself had destroyed. Was defeated by his son, Solid Snake and went into hiding until 4 years later when he became general commander of Zanzibar land. Was defeated by Snake in hand-to-hand combat.

Big Boss is the main character of Metal Gear Solid 3: Snake Eater, and Metal Gear Solid: Portable Ops.
His birth name is Jack.
Jack was born sometime in the 1930's (Date Unknown). In the early 1950's Jack joined the Green Berets and became The Boss's apprentice (The boss is hero from WWII). In 1954, Jack was involved in the first airborne detonation of a hydrogen bomb. The Boss taught him everything about combat, from weaponry, survival, espionage, destruction, psychology. She taught him to speak Russian as well. He and The Boss developed a combat technique called CQC (Close Quarters Combat). His mentor, friend, and lover, The Boss left for unknown reasons on June 12 1959.

On August 24th, 1964, Jack was part of an operation entitled the "Virtuous Mission", in which he was to rescue a defecting Soviet rocket scientist named Nikolai Stepanovitch Sokolov. It was during this mission that Jack was given the codename "Naked Snake." Naked Snake deriving from the fact that he was there with no weapons, food, or backup assistance, he was "Naked" just as him name implied. "Snake" came from the fact that he was a student of The Boss which was a member of the COBRA unit, a unit responsible for the Allies victory in WWII.
However, this mission failed, due to the defection of The Boss. It is later discovered that The Boss had not truly defected, but that she was acting as a double agent to retrieve the Philosophers' Legacy (A large amount of hidden wealth) from Volgin.
A wee...

The main character of Metal Gear Solid 3: Snake Eater. In the 70s he falls in a coma, so the pentagon takes his DNA, and they create three clones. (Solid, Liquid and Solidus Snake --> He is the youngest, but seems the oldest)The project was called "Las enfants terribles"(this means the terrible children and also was called Eve project) In the 80s he losses his eye and goes in retirement, then he trained other soldiers. He also becomes the of Fox-Hound.
He simply was the best solider who ever lived.