Metal Gear Acid Codes -

Viper card
Enter VIPER as a password to get a card featuring Black Arts Viper from the Metal Gear Solid on the Game Boy Color.

Jehuty card
Enter JEHUTY as a password to get a card featuring Jehuty from Zone Of The Enders. This card will destroy all equipment and equipment level-ups in the current room.

Karen card
Enter KAREN as a password to get a card featuring Karen from Policenauts.

Mika card
Enter MIKA as a password to get a card featuring Mika from Snatcher.

XM8 card
Enter XMEIGHT as a password to get a weapon card with 50 attack, 60% hit, 5.56mm bullets, 6 shots, 70% reaction, move 6 spaces, knockback, and burn.

Play as Teliko
Successfully complete the Residential Zone mission.

Solid Snake (MG) card
Successfully complete the game.

Raiden card
Successfully complete the game two times.

Link battle option:
Successfully complete Stage 06 to unlock the "Link Battle" selection at the "Options" menu under the "Intermission" screen.