Balloony is a recurring "character" of the animated series, Phineas and Ferb. It/He is a balloon that Doofenshmirtz acquired as a child. He drew a realistic face on it and made it his companion.
Balloony was Dr. Heinz Doofenshmirtz's best friend back in Gimmelschtump. It was lost one night when it floated away, and an attempt by him to grab it was thwarted by his father.
Balloony was then found years later aboard Mitch's poaching ship as one of the creatures he captured illegally. Mitch had renamed it "Colin" and called it his best friend. Heinz argues that Balloony was his best friend. Doofenshmirtz tells Balloony to come out of the ship with him, but Balloony didn't budge. This upsets the evil scientist and he says that Balloony has changed. He then realizes that his real best friend is Perry the Platypus. Just as he starts to say this aloud, Agent P jumps up and punches him backwards into the hole in the floor, causing him to fall to his doom. Perry then jumps out after Doofenshmirtz to save his nemesis. ("The Chronicles of Meap").
Balloony was later referenced when Doofenshmirtz won the Inator contest. His prize was a small red balloon, which he named Balloony 2 ("Robot Rodeo"). Later, Mitch and Doofenshmirtz find Balloony now equipped with a self-controlled robot suit. Balloony at first tries to defeat Doofenshmirtz, but when Doofenshmirtz (along with Perry the Platypus and Peter the Panda) is taken down by robots, he pleads with Balloony who has a change of heart and defeats the robots. Doofenshmirtz is very happy, and begins to tell Balloony about all the good times they can have together again, until Balloony gets hit by a shot from one of the fallen robots and dies (or pops) to Doofenshmirtz's extreme dismay. Some time later, Perry enters Doofenshmirtz Evil Incorporated where a miserable Doofenshmirtz tells him that he's got no scheme to be foiled as he was still down, only to realize that Perry had brought him Balloony newly inflated (with a band-aid) back to him, finally resolving the backstory. Doofenshmirtz and Balloony then have a really good time together making up for all the time they were apart. ("Meapless in Seattle").
While hosting the clip-show about Doofenshmirtz's life, Norm brings up Balloony. Doofenshmirtz was glad to see Balloony but it soon pops. Doofenshmirtz shouts in terror until Norm tells him that it was not the real Balloony and the real one "popped 3 weeks ago" ("This Is Your Backstory").