Big Boss/ Naked Snake

Big Boss is the original "Snake" and is the person responsible for starting it all! 

We first meet Big Boss in the original Metal Gear where he serves as Solid Snake's mentor and aid over the radio.  Later on in the game, we learn that he's actually the leader of the terrorist group in Outer Haven.  It is then Solid Snake's job to defeat him and to stop him from using Metal Gear to launch a nuke.

However, that's all later on in the timeline.  In the Metal Gear timeframe, we first meet Big Boss by the name of "Naked Snake."  As part of the Fox unit, his first mission in Metal Gear Solid 3: Snake Eater is to infiltrate the enemy base in the Soviet Union and to rescue a soviet scientist named Sokolov.

After the events of Metal Gear Solid 3: Snake Eater, Naked Snake earns the title of "Big Boss."