Green Circle is a passenger on the Space Symphony ship in Space Channel 5 Part 2. He's just one of many people in Extra Report 1 being shown "how to boogie" by the Rhythm Robots under Shadow's orders. Ulala must save him from being teleported away with the Rescue Beam when he's revealed.
“A nice young man who looks outstanding in the ancient garment “Kimono”. He comes from a well established family who are teachers of the Venus School of Tea Ceremony.”