In Pokemon Gold/Silver/Crystal, there are no key cards. However, there are Key Items that are essential for progression. Here's a breakdown:
* GS Ball: Found in the Ilex Forest, it's used to unlock the Legendary Beasts.
* Silver Wing: Received from a trainer in Mahogany Town, it's used to unlock the Legendary Birds.
* Secret Key: Found in the Ruins of Alph, it's used to unlock the entrance to the Hidden Grotto.
* Red Scale: Received from a trainer in Ecruteak City, it's used to unlock the entrance to the Ruins of Alph.
* Clear Bell: Found in the Radio Tower, it's used to unlock the Legendary Beasts.
* Rainbow Wing: Received from a trainer in Goldenrod City, it's used to unlock the Legendary Birds.
* Blue Card: Received from a trainer in Cianwood City, it's used to unlock the entrance to the Lake of Rage.
* Yellow Card: Received from a trainer in Cianwood City, it's used to unlock the entrance to the Lake of Rage.
* Green Card: Received from a trainer in Cianwood City, it's used to unlock the entrance to the Lake of Rage.
* HM01 Cut: Used to cut down trees.
* HM04 Strength: Used to move boulders.
* HM05 Flash: Used to light up dark areas.
* HM06 Rock Smash: Used to break rocks.
* HM07 Waterfall: Used to surf up waterfalls.
* HM08 Dive: Used to dive underwater.
* HM09 Surf: Used to surf on water.
* HM10 Fly: Used to fly to previously visited locations.
These are just some of the Key Items in Pokemon Gold/Silver/Crystal. Make sure to explore and talk to trainers to find them all!