After SpongeBob caught it, the Jellyfish followed him, which SpongeBob tried to get rid of, but finally he decided to keep him. He knows how to fetch, count, and play dead. He is also "potty trained" reading a newspaper. Squidward is annoyed by him. He enjoys dancing with SpongeBob.
After escaping his leash at night, he invited almost every Jellyfish in Jellyfish Fields at a dance party. They also broke Squidward's clarinet and put it on fire. After SpongeBob accitently broke the music player, they attacked him and Gary but Gary started playing music knocking his eyes each other, along with other animals playing music, leading them back to Jellyfish Fields, including Jelly.
He's just an ordinary pink jellyfish with spots.