The Sorrow

The Sorrow (ザ・ソロー) was 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.

Biography
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 (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 (at his request) during a covert mission at Tselinoyarsk in 1962. Before he died, he told her "the spirit of the warrior will always be with her."

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 game's first-person view cinema sequences, where he can be seen observing Snake and the events of the mission, and occasionally offering helpful hints, such as 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 Snake face the spirits of every single person he killed during the game. The spirits of the fallen Cobra unit members also appear. The Sorrow also tries to project images into his mind with an attack, which is foreshadowing of The Sorrow's past and his death.

The next time he is seen helping Snake by showing how much time is left before the C3 destroys the Shagohod hangar. It is also 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.

The Sorrow was last seen near the end of Operation Snake Eater, standing next to the ghost of The Boss, reunited and vanishing together.

Guns of the Patriots
In 2014, the Sorrow makes an appearance as one of Screaming Mantis puppets. His puppet gives Screaming Mantis control over dead FROGS in the vicinity. It's not until after Snake defeats Screaming Mantis, that The Sorrow makes a short appearance as a ghost, reciting his last words that he said to the Boss before she killed him, "The spirit of the warrior will always be with you." He might be the reason for Psycho Mantis' ghost suddenly disappearing, possibly banishing him back to the spiritual realm.

Trivia

 * 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 the CQC choke, and eliminates footstep noise.
 * 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 Metal Gear Solid 3: Subsistence, if the free camera is on, the player can also see the skeleton by rotating the camera before going into the menu to cure Snake's wounds;
 * After The Boss is defeated, holding R1 and looking to the left allows the player to see both The Sorrow and The Boss's ghosts.
 * During the "fight" against The Sorrow, everybody contacted by Codec will all be screaming "Snake!" as if the player has just died. This includes Para-Medic, although the player can still save.
 * If the fight lasts long enough, The Sorrow will exclaim "you will be killed by your own sons."
 * Despite "The Sorrow" being his codename, The Sorrow is always smiling, with not one scene where there is a frown on his face. This is simillar to The Joy who is rarely seen smiling.
 * In MGS4, there is a secret R1 scene during The Sorrow's cry of "the spirit of the warrior will always be with you." The Sorrow will be visible floating behind Psycho Mantis.
 * If the player saves the game while in the fight with The Sorrow, their location will be listed as "?" (Known as "The Other Side"/"The World of the Dead" after the Boss Fight and contacting the Major)
 * If the player pays attention to his equipment, they'll see that he has a handgun in a cross-draw M6 shoulder holster, although it is always unused.