Petey Piranha appears as the first boss in Super Smash Bros. Brawl's Subspace Emissary mode, holding the cages of both Princess Peach and Princess Zelda. The player must destroy one of the cages (attacking Petey's head splits the attack's damage to both cages). When Petey is defeated, whichever cage is more damaged will break, freeing the princess inside. The princess who was not saved will be turned into a trophy by Wario. At the last level, Tabuu resurrects Petey (along with the other bosses) and sends him after the heroes in "The Great Maze."
Unlike in most Mario games, where Petey has a cartoonish voice, Super Smash Bros. Brawl has him using fearsome roars. This could be the result of Tabuu's power, or just to make him more threatening (the same has been done to Bowser and the Kongs).
Petey has only two main attacks. The first involves swinging one of the two cages along the ground. The second is a jumping attack, where Petey tries to stomp and bite the player. Only contact with Petey's head will damage the player when he jumps and stomps.
When either cage is reduced to half health, Petey will roar and his head will glow red momentarily. Petey will then speed up his attacks.
A gigantic Piranha Plant. Unlike his pipe-inhabiting cousins, he uses his legs to walk around. He can fly, too. As you might expect, he often appears as a boss character in Mario's adventures. This may come as a surprise, but he also displays surprising dexterity in kart races and ball games.
If one breaks both cages simultaneously by only attacking Petey's body, the game will randomly decide which princess is freed.
In the Great Maze level of the Subspace Emissary, and Boss Battles Mode, Petey's cages are empty and the background is changed to daylight.
During the cutscene before the first fight with Petey, a cannonball KO's Mario and knocks him out of the arena. It is never made clear where this cannonball comes from; Wario's Dark Cannon can't produce cannonballs, Petey Piranha himself is incapable of producing cannonballs in any game, and the Halberd's cannonballs move in a swirly motion, are blue, and explode on contact. A theory might be someone put the cannonball in Petey's mouth and Petey spat it out and due to his immense mouth it could've had enough force to knock Mario away. This theory is supported by the fact that when the part that shows the cannonball come out, if one looks closely in the smoke, a silhouette of Petey can be seen.
Petey's defeated animation causes him to catch on fire, no matter what move was used to defeat him.
Almost comically, if one redoes the fight from the first level against Petey using Peach or Zelda, they will still be seen in the cage with the exact same costume.
If one looks closely at the cages in the cutscene "Princesses, Both Captured and False", the cages that the trophies of Peach and Zelda are in are the exact ones Petey used.
Petey Piranha is the only boss to have two health bars. He is also the only boss that does not have his name displayed next to them. In the Subspace Emissary, the names displayed next to the health bars are Peach's Cage and Zelda's Cage and in Boss Battles, the word "Cage" is displayed next to both health bars.