Mr. Green is a friendly monster from the episode Substitute Creature, filling the role of substitute teacher, when Ms. Keane couldn't come to school that day due to illness.
He is introduced with a dark and eerie entrance. With a creepy, monstrous figure, the girls assume that he is a dangerous villain. They also assume that he has done something to Ms. Keane when he announces that she's sick and he is the substitute.
But Mr. Green quickly proves to be kind, honest, generous, and a competent teacher. The girls, still thinking that he is planning on doing something awful, become paranoid. Every little thing he does or says sends the girls imagination into wild anime-esque horror as they imagine that Mr. Green is about to do something awful. Eventually, Mr. Green decides to keep them inside for recess as a punishment for their behavior.
The girls finally realize that he really is a very nice monster who loves children and enjoys teaching, when their classmates vouch for him right as they are about to attack him. They apologize to Mr. Green saying that they have learned their lesson to not judge someone by the way they look. Mr. Green accepts the apology and reveals that he thought Powerpuff girls were just cute ordinary girls and not superheroes.
He appears to be somewhat of a male version of Maleficent. His appearance is similar to the members of the Gangreen Gang, apart from the fancy clothes.
His head is long and thin, and his ears and chin are pointed. He has green skin, long white hair, large gray ram-like horns that curl backward slightly, red eye(s), short nostril slits, sharp teeth and claws, and a scar under his patched eye. Other than that, he appears to be humanoid. He wears an eye-patch over his right eye, a dark gray tunic-like supervillain suit, a flaming burgundy cape, and black gloves.
- When Mr. Green first appears, his eye is bright red, but throughout the rest of the episode, it is black.