Hes bills friend on island 1
You find the ruby and give it to celio
You need the rainbow pass to get there. That means you must help Celio finish his machine.
It is in cereulan city. You can only go there before you beat the elite 4 and bring the ruby and sapphire to celio. You can catch mewtwo there.
Celio is a very useful character, upon complete of the task he gives you (after you've completed Islands 1-3 , beat the Elite 4 and gotten the National Pokedex) you can trade with Ruby, Sapphire and Emerald.
you must have given the ruby to celio on island one he will give you rainbow pass which lets you go to islands 4-7.
You find the ruby and give it to celio
Celio is the man standing right by the giant machine in the One Island Pokemon Center you can't miss him.
He is on 1 island in the Pokemon center building in front of the huge machine
celio is on the first island you can go there after you win at the cinabar gym, bill will be wating for you outside the gym
You also find the sapphire and give them both to Celio.
beat the elite four
to get to ruin valley on Pokemon leafgreen you have to get the ruby in mt. Ember and bring it back to celio then go to six island
you find him on one island in a building with a pokeball on it and a red roof.
On five island you need to have given the ruby to celio to get there.
He is in the one island Pokemon center he's the guy with the glasses near the big machine.
beet the elite 4 talk to celio and then you can go to all the islands
you get it by finding the ruby in mount ember and giving it to celio in the pokecenter