Balloons

The Balloons make their first appearance in "Ricardio the Heart Guy." They fly up on screen in this episode when Princess Bubblegum says that she'll throw a party for Finn and Jake for rescuing her from Ice King once again. The screen says "PARTY TIME!" and the balloons fly up.

They appear again in "What is Life?," and they have more importance in the episode. In "What is Life?," it is mentioned that they are great pranksters. One of them said that he stole Finn's wallet; This seems impossible because the balloons have no arms. The balloons apparently had a blood oath with Finn. Finn releases them from it when they transport him to the Ice Kingdom. After that, they float to the mesosphere to die, which they are excited to do. Before they die, they go on many adventures, as stated by Adam Muto.

They also make a cameo in the beginning of "The Duke," when Finn and Jake throws bottles of magic at a wall.

In the episode "Marceline's Closet," Finn says he likes to make music from balloons and can be seen making some.

Appearance
The balloons are all similar-looking with a variation of colors. The one exception is a balloon that has an angry-looking face, compared to the others which have simple faces.

Trivia

 * The purple balloon resembles Eight-Armed Willy from The Marvelous Misadventures of Flapjack.
 * The balloons may be suicidal, as they seemed happy with the fact that they would die when they reached the mesosphere.
 * The dark purple balloon seems to have the most detailed face.
 * In What is Life, It Is Revealed by Finn That The Balloons Are Grade-A Pranksters

Quotes
"We got your back!"

"I got his wallet!"

"To the mesosphere! Finally we can die!"