Master Chief (real name John-117) is the last of the Spartan II warriors - an elite group of genetically superior, highly trained super-soldiers.

Spartan 117 is expert in all weapon and combat systems. He can run faster, jump higher and hit harder than any normal human. He is the perfect weapon, honed by years of battlefield experience and possessing a natural talent for war, unmatched by anyone left alive. He is our last and best hope.

NOTE: Here we see again the Chief referred to as the "last Spartan" even though events in the novels would indicate that other Spartans are alive and active elsewhere in the Halo universe. Given that not everyone reads the novels, it seems that this is done to underscore the Chief's uniqueness within his own context rather than to contradict the novels directly. Given how consistently this has been done since Halo 1, it seems reasonable to assume that other Spartan IIs will likely not appear in Halo 3, either as playable characters or as NPCs.