Rusukaina is an island in the Calm Belt that is towards the northwest of Amazon Lily. This island is harsh to visitors because of its weather as it had forty-eight seasons a year which change on a weekly basis.
Rusukaina is known for its harsh climate. There are also a lot of dangerous wild animals that live on Rusukaina. Boa Hancock herself said that this island is very dangerous even for the Kuja warriors. The only safe area on this island is a tree that all the animals avoid. This tree is like a Daft Green but doesn't emit poison.
According to Rayleigh, when Luffy arrived on the Island there were over five hundred beasts living there as he wouldn't be able to defeat unless he got stronger. After the timeskip, Luffy was able to master Haki and completely conquer the island.
Rayleigh took Luffy here to train him here and make him learn how to use Haki. Rayleigh spent the first year and a half teaching Luffy the basics of Haki. After Luffy mastered them enough, Rayleigh left him alone for the remaining year and a half to finish his training and it was during these two years that he developed Gear 4 to deal with the tougher animals.
After another half of the year of training, Luffy became the boss of the Island and even befriended some animals. He left the island when it was time to rejoin his crew.
Rusukaina is a play on the phrase which roughly means 'is nobody home?' or 'nobody's home, huh?'
Rusukaina had people living on it but they all couldn't survive against the extreme environment and died out.
Silvers Rayleigh was the one who brought Luffy to Rusukaina.