He is introduced with a dark and eerie entrance. With a demonic figure, the girls assume that he is a wicked monster. 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, 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. He's basically a cross between a devil (hence the horns and stature) and a witch (hence the skin color).
His head is tall and thin and his ears are pointed. He has green skin, white hair, large gray ram horns that curl, short nostril slits, sharp teeth and claws, and a scar under his eye. Other than that, he appears to be humanoid. He wears an eyepatch over his right eye, a dark gray supervillain suit, a flaming burgundy cape, and black gloves.