She was voiced by Sarah Hyland.
So You Think You can Stank
In this episode she competed in the talent contest, however her performance was very rudely insulted by Howard along with the others. Overcome with humiliation, she became the victim of the Sorcerer's stank, and was turned into a monster upon the stank exposure. She went on a rampage, Randy took her on, and managed to turn her back to human.
Der Monster Klub
Following Randy accidentally revealing the secret of turning into a monster, Julian quickly took advantage of this, by exposing an embarrassing secret about her (which implied once taste something that highly disgusting), overcome with humiliation and sadness, she was turned into a monster. She went on rampage, kidnapping the principal of Norrisvile, as he had canceled the baton twirling team which she loved(which serves as secondary ). Randy took her on and managed to turn her back to normal, along with the reminder of the Der Monster Klub.
Stank Like Team Spirit
Following her (and the rest of the team) loosing the annual chess tournament to Hannibal McFist's chess robot, overcome with grief and anguish at losing, she (along with the rest of the team, and several other students) was transformed into a monster. In this form she went on a rampage, Randy managed to temporarily stop her, along with the other monsters, however and she was changed back to normal, when Howard defeated the robot.
Shloomp There it is!
After heart broken by Nomi-Randy, (who she believed was real Randy) once again fell victim to the Sorcerers Magic and was once again turned into a monster. Overcome with fury, she chased after Heidi Wienerman trying to kill her for revenge. She was turned back to normal, when Randy, dressed as the Ninja gave her a bouquet of roses, claiming they were from Randy to say he was sorry. This is the only known circumstance when her behavior pattern match that of rampaging Yandere.
Winner Takes Ball
Though technically not under conventional stank influence, Theresa ended up taking an antagonistic role alongside Norrisville citizes except Randy and Howard due to drank the contaminated water and is brainwashed by Evil Julian: After Howard wasted one of the Chaos Pearls' dark powers (which the pearl's dark powers revealed to be stored inside the pearl in the liquid form), he accidently dropped the pearl to the street where it eventually crushed under their school bus' wheels. With the pearl crushed, the very small amount of the pearl's liquified dark powers poisoned the sewers and later, the city's power supply. Due to the dark magic poisoning on the water is supernatural in nature, it remains undetected until it's too late. One by one, Norrisville citizens whom drank the contaminated water fall under the dark magic influence and later fall into Evil Julian's Under Control. This includes Theresa, whom later goes to Randy's room much to his awkward due to the fact that she was a girl whom supposedly not entered a boy's room. She demaned Randy to give her the ball, but Randy awkwardly denies, resulting her attacking him. He eventually forced to locked her into his wardrobe until the end of the episode where after Evil Julian absorbs the dark magic influence that corrupt their mind, Theresa' corruption ws undone and she was changed to normal.
- Theresa surely bears a resemblance to Sam Manson from Danny Phantom.