I'm afraid you cannot get a Celebi in Pokemon HeartGold, unless you use a Action Replay. The only Celebi legal, I think, is the Nintendo Event that has passed. If you do have the Celebi in your GameBoy Pokemon Cartridge, then you can migrate it to Pal Park, then trade from Pearl/Diamond to HeartGold/SoulSilver.
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Get the celebi event.
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No, unless you want to use the Action Replay for the Event.
You don't. He's not there. Only in HeartGold and SoulSilver with the Celebi event.
You can get a Spiky-eared Pichu in Pokémon HeartGold and SoulSilver if you get the movie-event Pikachu-colored Pichu and bring it to Celebi's Shrine in Ilex Forest in HeartGold or SoulSilver.