You find the ruby and give it to celio
you can go and catch mewtwo
Hes bills friend on island 1
Celio is the man standing right by the giant machine in the One Island Pokemon Center you can't miss him.
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
beat the elite four
You also find the sapphire and give them both to Celio.
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
On five island you need to have given the ruby to celio to get there.
you find him on one island in a building with a pokeball on it and a red roof.
you get it by finding the ruby in mount ember and giving it to celio in the pokecenter
beet the elite 4 talk to celio and then you can go to all the islands