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91C089DA. You can use GameShark or other similar devices. Then talk to Kurt. Before you insert the code, make sure that Kurt is not checking any Pokeball already. Kurt will automatically give you a GS Ball, which you can use right away in the Ilex Shrine to capture Celebi. Note: You can use this code as much as you want, but keep in mind that there will be a few resets whenever you do this. There will always be a reset whenever you use the codes for Celebi. In the Dragon Shrine, the master will give you another Dratini for unknown reasons but can easily be fixed by talking to him again. Just accept the Dratini, and he will be back to normal. Another one is the girl in front of Sudowoodo. She will be standing there like before, and you have to talk to her to make the girl go away. The man you encountered before who explained that there was something odd in the Underground Chamber that made him flee will say the exact thing again, as if it's the first time talking to you. To fix this, just talk to him once more, and he will be back to normal. Aside from those resets, nothing majorly changed in the game. After you obtained Celebi, make sure to save your progress.
Insert this code: Fix the resets.