He bears a strong resemblace to Frankenstein's monster.
Most of the time he is rude and very stern with the students but deep down he cares about (most of) them.
She is Frankengymteacher's girlfriend. They are very much in love with each other. It is revealed in The Last Dance that he had a crush on her in his school days and bought a pair of dancing shoes for her but those lay abandoned in a closet at Scare School until Mantha tries them on and gets them stuck to her feet forcing her to dance until she lets Hedy borrow her legs so she could dance with Frankengymteacher in them.
Frankengymteacher seems to dislike Thatch as he stole Thatch's sandwich and ate it in front of him when he thought he could turn invisible in "Now You See Me Now You Don't".
FGT apparently dislikes Mickey and Monaco since he painted mustaches in their faces with their lipsticks in "Now You See Me Now You Don't".