The Green Guise is the level 30 Thief hat

 Stats:
 Req Level: 30
 Req Dex: 60
 Req Luk: 80
 W. Def: 19-23
 Luk+1
 Slots: 7

You can purchase it for 30,000 Mesos at the Kerning or Orbis armor stores.

Drops from: Ratz and Retz