Master Green is a big sea jelly imbued with electricity. His head is dome-shaped with two small black eyes. From beneath his head, eight thick tentacles hang down. His official artwork for Kirby's Dream Land 2 depicts him as yellow, despite his name. However, his appearance in Kirby of the Stars Pilot and the artwork from Kirby 25th Anniversary Twitter depicts him as green. Although he is a jellyfish, he shares more traits with cephalopods than cnidarians. Master Green is also not transparent like regular jellyfish.
When encountered, Master Green attacks by skidding across the ground and jumping around, eventually producing sparks from his body (like a Sparky), shooting sparks at Kirby, or sending an electric shock wave across the ground. He always slides or jumps before the shock. After the shock wave, he throws an electrical star towards Kirby that explodes, which he can inhale and spit back at Master Green. He can also jump into the air and throw stars from above.
Master Green appears in the Kirby of the Stars Pilot along with Captain Stitch, Meta Knight, Ice Dragon, and Dark Matter as one of the monsters that were summoned by Nightmare from the darkness. Kirby defeats monsters and eventually inhales Master Green, gaining Spark ability from him. He uses it to destroy Efreeti, Blocky, Haboki, Sweet Stuff and Mr. P. Umpkin.