Enter the following codes at the Password menu to unlock the corresponding characters.

Unlock Cunningham:
JUNKER

Unlock Elisa:
THE-L

Unlock Eva:
E.APPLE

Unlock Ga Ko:
!TRAUMER

Unlock Gene:
ERBE

Unlock Null:
Hunter-n

Unlock Ocelot:
R.R.R. Unlock Paramedic:
PM-EMS

Unlock Python:
LQ.N2

Unlock Raikov:
IVN =RV

Unlock Sigint:
DARPA-1

Unlock Sokolov:
SATURNV

Unlock Teliko:
T.F-ACID

Unlock Ursula:
PK +ESP

Unlock Venus:
MGA2VE

Unlock Zero:
1+2-3

Unlock Lieutnant Cunningham:
On any difficulty defeat Cunningham by 'Stamina Kill' and beat the game before January 1st 1971. Cunningham will then be unlocked when you make a new game.

Unlock Null:
Defeat Null twice by either 'Stamina kill' or 'HP kill' and then beat the game. Null will then be unlocked when you make a new game.

Unlock Para Medic:
On the Comm Tower stage contact her on the radio then later in the Hospital stage rescue her. Place spies in the Hospital.

Unlock Raikov:
Place spies in Western Wilderness then clear the Guard House mission. You will then get a report of a prisoner being held there.

Unlock Sigint:
When Snake has been captured and rescued you can contact Sigint on the radio at the Comm stage then rescue him in the town.

Unlock Major Zero:
Have a Metal Gear Solid :DGN game save on your PSP's Memory Stick (Second Playthrough only).

Unlock Teliko:
Have a MGA game save on your PSP's Memory Stick. (Second Playthrough only)

Unlock Venus:
Have a MGA 2 game save on your PSP's Memory Stick. (Second Playthrough only)

Unlock Gene:
In one playthrough recruit 200 soldiers and 'Stamina' kill Gene.

Unlock Sokolov:
Give the Saturn V blueprints to Sokolov

Unlock Python:
Kill Python by 'Stamina kill'.

Unlock Elza:
Get a Medic level of 80.

Beat the game with your Technician status level at 99. The stealth camo will let you become invisible for a limited time.