The "Sirran the Lunatic" that spawns after you clear the 6th island (Bzzat and Co.) in plane of sky is the Sirran from the 5th island. (You can tell by hailing him, his response is the same response from the 5th island... not the 6th island Sirran who talks backwards to you.
This specific one is necessary to spawn Sister of the Spire for quest loot and such with a phrase... not just for the key (which you don't need, obviously).