Answer 1

If you take an event celebi to the llex forest. It will take you 3 years back in time, where you discover that your rival is the son of giavonni, that is why he hates team rocket. You also get to battle giavonni.

more info here

http://www.serebii.net/heartgoldsoulsilver/celebi.shtml

Comments

14 seconds too late.

Answer 2

If you take it to illex forest you battle ginavvi.

Comments

(In sonic the hedgehog voice)  You're too slow! one second.

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Go Super Smash Bros Brawl. Metaknight for the win.

Answer 3

After getting Celebi, go to Ilex Forest. You will go back in time and find out your Rival is Giovanni's son. Then you go foward a bit to the Radio Tower takeover. You find yourself in a cave and facing Giovanni.