The Sorrow

The Sorrow appears in the game Metal Gear Solid 3: Snake Eater. He is the "Spirit Medium Soldier" of the Cobra Unit.

Although he has no actual unique combat abilities like the other Cobras, The Sorrow is gifted with the powers of a medium. He is able to summon the dead and assume their combat abilities by bringing their spirits into himself. Communicating with dead soldiers also gives him several advantages during a battle, such as learning of enemy positions. Like the other Cobras, his codename reflects the emotion he brings to the battlefield.

History
The Boss and The Sorrow fought together as part of the Cobra unit during World War II and participated in the Allied landing at Normandy in 1944. The two had a child together (Revolver Ocelot), who was taken from them by The Philosophers. After the War, he worked for the Soviet Union and was ultimately killed by The Boss during a covert mission at Tselinoyarsk in 1962.

In 1964, The Sorrow appears to Naked Snake as a ghost. Several times, he is seen crying blood. Whenever he does, rain falls from the sky. He frequently appears during the games' first-person view cinema sequences, where he can be seen observing Snake and the events of the mission, and occasionally offering helpful hints, like a key radio frequency.

When Snake meets The Sorrow in the spiritual realm after jumping off a water fall, he is forced to walk down a seemingly endless river, so that he will "feel the sorrow of those whose lives he ended". He makes him encounter the ghost of every single person killed during the game. The ghosts of fallen Cobra unit members also appear. The Sorrow also prophesied that Big Boss would be killed by his own "sons" (the results of the Les Enfants Terribles project).

It is theorized that The Sorrow was responsible for summoning the lightning that killed Colonel Volgin, because in the scene when Volgin dies, when EVA is about to hug Snake, if the player holds R1, they will see The Sorrow floating behind her and smiling.

If the player makes it to the end of The Sorrow's river and uses the revival pill, when Snake gets his gear back, he will have obtained the Spirit camo, which allows Snake to drain stamina using CQC, and eliminates footstep noise.

Behind the Scenes

 * The Sorrow's corpse (or rather, his skeleton) can also be seen during a secret R1 scene after Snake is thrown off of the Dolinovodno bridge, after Snake bandages himself.
 * In Guns of the Patriots, The Sorrow makes a semi-cameo appearance as one of Screaming Mantis' puppets. His Puppet gives Screaming Mantis control over dead FROGS soldiers in the in the vicinity. Shooting off the Sorrow doll will allow you to pick it up. If fired at a dead body, you can use SIXAXIS plus L1 to get off items from that body.
 * In Guns of the Patriots, The Sorrow causes it to rain right as Liquid Ocelot takes control of the SOP system. The rain symbolizes the emotional low of Snake's and Eva's defeat,  Liquid Ocelot taking control of the world, and Solidus Snakes true death at Liquid Ocelots hands during the end of ACTIII.
 * After defeating Screaming Mantis, and the demonstration of Psycho Mantis' power from his spirit, The Sorrow repeats a line from the cutscene in Metal Gear Solid 3: Snake Eater where the first few minutes are an audio flashback of his last moments before being killed by The Boss. If you hold the L1 button when the button prompt shows up, you can see The Sorrow's spirit floating in the upper left area of the command center on board Outer Haven.