Capture 3 specific Pokemon characters and confirm you've got 3 additional in your roll of characters. You’ll need Suicune, Raikou, Entei, Articuno, Zapdos and Moltres.Move your character up to the lady and press to talk. once you’re done speaking, the lady can provide you with a special gift: a GS ball.
Head to the Ilex Forest and look for the shrine there. Carry your ball inside the shrine. As soon as you are doing this, Celebi can embark, and you'll be able to capture the character.