The cyborg ninja encountered by Solid Snake in Metal Gear Solid is Gray Fox.
Gray Fox is the codename of Frank Jaeger, a former FOXHOUND operative and an exceptional soldier. During the Shadow Moses incident, he appears as a technologically enhanced ninja who first attacks Snake and later helps him. His high-frequency blade and optical camouflage make him one of the game’s most memorable figures.
The character’s history reaches back to earlier Metal Gear games, where Frank Jaeger served as a soldier connected to Big Boss and Solid Snake. Players sometimes answer Raiden, who becomes a cyborg ninja in later entries, but Raiden is not the ninja encountered in the original Metal Gear Solid. Vamp is another enhanced combatant in the series, yet he belongs to Metal Gear Solid 2 and Metal Gear Solid 4 rather than this encounter.