Go into the Pokemon mansion and look around. At the end you will find the key to the door.
well first after you beat the 7th gym leader bill takes you there to the islands
I think its when you get back from the islands he appears as the gym leader of veridean
The fifth Gym Leader in Pokemon LeafGreen is Koga, the leader of Fuschia City's Gym. He specializes in Poison-type Pokemon and hands out the Soul Badge.
govinia
in fuchsia city
From the leader Koga in Fuchsia City
beat the gym leader
In Fuchisa city
Her name is Erika
On cinnibar island.
you get it at safron city gym leader
Veridian city