Miasmo has a large grey body made of smoke that ends in a point, with a black face and hands. His face is similar to that of a Smogling, with diamond shaped blue eyes, a triangle nose, and a wide smiling mouth with 2 points on the bottom. His hands and face have a smokey look to them as well.
Miasmo is a cunning and manipulative villain. He is merciless and will not hesitate to get rid of something if it doesn't meet his standards. He laughs often, usually resulting in coughing. He shows that he will stop at nothing to achieve his goals.
Miasmo is revealed partway through the game to be the one who is manipulating General Greenthumb. He appears to Greenthumb after flashback scenes in order to force him to turn back into Sergeant Smogglor, surrounding him with a black smoke, and upgrading his ZAPOW attack to use on Chibi-Robo. He also allows Smogglor to use the Smogglob. Once the park reaches a certain amount of flowers, he attacks directly, using his Toxic Storm Cloud attack to turn many flowers into grey Miasmo Flowers. After the third flashback, he decides Smogglor is of no use to him, calling him trash and abandoning him at the park. Once the park reaches 999 flowers, Miasmo will send a group of Smoglings to bring him to Exhaustia for the final fight. Once he is defeated, he claims that he can never truly be stopped as long as humans continue their current ways.
Miasmo will begin the battle by summoning groups of Smoglings against Chibi-Robo. Defeat the Smoglings to spawn buds, which can be watered and grown into flowers. Growing a flower causes damage to Miasmo. Occasionally during the battle, Miasmo will use his Toxic Storm Cloud to turn flowers into Miasmo Flowers, restoring his health. If a Smogling turns a flower black, it will also restore his health. Eventually, he will begin to summon Smogglobs, which will create many buds if defeated. Once he has ten health left, General Greenthumb appears, leaping onto Miasmo and tickling him to prevent use of Toxic Storm Cloud. Once his health is reduced to zero, he explodes into rainbow colors, before vanishing completely, sending Greenthumb and Chibi-Robo back to the park. Following his defeat, all Smoglings will turn pink, becoming friendly.