First appeared in Metal Gear Solid 2: Sons of Liberty, RAY was designed by the Marine Corps as anti-Metal Gear weapon. Compared to other Metal Gear models, RAY was light-weighted and versatile, opposite of what REX was. Since RAY was developed by the Marine, RAY can go underwater whenever it wants to, giving it a huge advantage.
RAY's main weapon is the hydro cutter that is installed within its mouth, just below where cockpit would be. The hydro cutter has more power than average one, enough to cut through any thick metal, just like the defense shell of REX, which was the real purpose of RAY's hydro cutter. RAY's other weapons include two machine guns located at the end of its wings and missile tubes located at its back.
Metal Gear Solid 2: Sons Of Liberty
When Snake and Otacon found out that U.S government was developing its own Metal Gear, Snake infiltrated into the Marine's ship to reveal the existence of it to the world. However, Snake was not the only one aboard the ship, Revolver Ocelot was also on the ship he had different purpose, he was there to take it. Ocelot Successfully do so and manages kill Sergei Gurlukovich who also had intention of stealing RAY for Russia but that was not the case for Ocelot. RAY has become Ocelot's personal weapon since.
However there was more than one RAY, there were 25 unmanned models with sole purpose of protecting Arsenal Gear.
Metal Gear Solid 4: Guns of the Patriots
Liquid Snake appears in MGS4: Guns of the Patriots again, but this time with RAY. RAY made its appearance again when Snake piloted REX, and the two fight against each other.