First appearance: Metal Gear (MSX2)
Looks like: Your Grandpa
Voiced by: David Hayter
Biography: Solid Snake is a black ops/spy entrusted with keeping the world safe from the huge bipedal, nuclear-armed robots known as the Metal Gears. Since the first Metal Gear, Solid Snake has gone from snot nosed new clone (reproduced from the DNA of Big Boss) to white-haired and decrepit war veteran (despite only being set five years after Sons of Liberty).
What he's doing in MGS4: was last seen melting into the crowd at the end of MGS2 with information on The Patriots - a sinister organization controlling the US Government. Despite having retired from active duty more times than "Sugar" Ray Leonard, he's now back to take them on - let's hope his colostomy bag is suitably hardy.
First appearance: Metal Gear Solid 2 (PS2)
Looks like: Freddie Mercury
Voiced by: Phil LaMarr
Biography: A dangerous mercenary with a taste for blood, Vamp was a member of the Dead Cell unit that was co-opted to become the terrorist organization "The Sons of Liberty" in MGS2. Vamp seemingly has supernatural abilities, as well as being indestructible - Raiden apparently kills him several times in MGS2, though he is seen in the crowd at that game's conclusion.
What he's doing in MGS4: MGS4 will see a rematch between Raiden and Vamp - the 2007 E3 trailer showcases some of this epic fight, and it's a doozy. Whose side is Vamp on though? Nobody is sure yet…