From Metal Gear - Wiki on Neoseeker

[edit] Unlockable Characters

The following hints and cheats are to unlock special characters in the game. For each character, there will be a campaign/game and password way to unlock them.

Cunningham: Beat Cunningham on any difficulty by stamina kill and before January 1st 1971 in the game. (Obtained in second play through)

Passwords: America = JUNKER , Europe = CUN=BOA

Elisa: Get your Medic level to 80.

Passwords: America = THE-L , Europe = E.WASP

Eva: ?

Passwords: America = E.APPLE , Europe = 016K/HYH

Gene: Recruit 200 soldiers and stamina kill Gene, all in one playthrough. (Obtained in second play through)

Passwords: America = ERBE , Europe = 9-VIPER

Major Zero: Have a MGS: DGN game save data on your psp's memory stick. (Obtained in second play through)

Passwords: America = 1+2-3, Europe = GALAXY-0

Null: Beat Null by stamina kill or HP kill when you face him twice and beat the game. (Obtained in second play through)

Passwords: America = Hunter-n , Europe = ALL=0

Ocelot: ?

Passwords: America = R.R.R. , Europe = THE-SON

Para Medic: Contact her on the radio on the Comm Tower stage. She will later need to be rescued in the Hospital stage. Must place spies in the hospital before hand.

Passwords: America = PM-EMS , Europe = E.M.S.

Python: Beat Python on any difficulty by stamina kill.

Passwords: America = LQ.N2 , Europe = GUFO

Raikov: Place spies in Western Wilderness before clearing the Guard House mission. Afterwards you should get a report of a handsome prisoner being held there.

Passwords: America = IVN =RV , Europe = SKAZKA

Sigint: After Snake is captured and rescued you can contact Sigint on the radio at the Comm Tower stage. He will need to be rescued in the Town stage. Must place spies in the Town before hand.

Passwords: America = DARPA-1 , Europe = D+ARPA

Sokolov: Give the Saturn V blueprints to Sokolov.

Passwords: America = SATURNV , Europe = VOSTOK

Teliko: Have a MGA game save data on your psp's memory stick. (Obtained in second play through)

Passwords: America = T.F-ACID , Europe = SHINE!

Ursula: Have a Medic level of over 80, with the second number as an odd number.

Passwords: America = PK +ESP , Europe = ESP:ASP

Venus: Have a MGA 2 game save data on your psp's memory stick. (Obtained in second play through)

Passwords: America = MGA2VE , Europe = @VESPER