Theresa is tall, with dark blue eyes. Her hair is blue violet with three lavender streaks, and she wears it in a ponytail with a band. She wears the Norrisville High Twirl Team outfit, a long-sleeved golden dress with blue and white stripes on the sleeves, the collar, around the waist, and the bottom. Her stockings are dark purple and white striped, and wears blue tennis shoes.
Whenever Theresa is stanked by the Sorcerer, she turns into a green, plant-like monster with tentacles for arms and legs. The two fingers at the end of each of her arms allow her to spin twirling batons or grab things. As a monster, Theresa is somewhat flexible, as she is able to jump through walls, and often while not moving her body is swaying. She seems to be elastic, reaching from a far distance to grab Heidi Weinerman, but missing and grabbing a locker instead in "Shloomp! There It Is!".
Theresa is kind, friendly and compassionate, having a considerably good-sized amount of friends at school despite being unpopular. She is also emotional, like when she was resentful of Principal Slimovitz in "Der Monster Klub" for getting rid of the school's baton twirling team and cries from heartbreak when Randy--who is actually NomiRandy--ignores her to flirt with Heidi in "Shloomp! There It Is!".
But Theresa isn't weak; she is practical and perceptive. While hiding from the zombies in a mansion during "Night of the Living McFizzles", she acts level-headed in comparison to Howard, giving McFizzles to him to make him stop crying. She isn't afraid to go for what she wants, like Randy, but she isn't desperate for it. In "Sorcerer in Love", she isn't depressed that Randy isn't dancing with her, but annoyed by Randy and Howard's arguing, which makes her walk away.
Whenever Theresa and Randy interact, they usually get along well. The two both think that Howard as the Ninja doesn't seem right in "The Ninja Supremacy" and talk about it, although Randy doesn't remember who she is at that time. They also seem to share similar reactions in particular events, both teens wide-eyed with horror when Julian humiliates Juggo and Dave in "Der Monster Klub".Most of Theresa's appearances have shown she has feelings for him. She is impressed with his lack of fear, calling him hardcore, and seeing him as her protector. When terrified in "Night of the Living McFizzles" and "Sorcerer in Love 2: The Sorceress's Revenge", she looks to him for safety and leadership, asking him about what they should do. To Randy's surprise, she also holds his hand during frightening moments, and occasionally gives him a reassuring or caring smile. She worries about him when he disappears to become the Ninja in the latter episode, glancing behind to check for him and asking Howard and the Ninja about him. She seems to believe that he is a hero, and often reacts to him positively. Her crush has grown to the point where she gets stanked from the shock and sadness of NomiRandy ignoring her and flirting with Heidi in "Shloomp! There It Is!", which made her cry from heartbreak which led her attacking Heidi in revenge. She is destanked by her happiness for the bouquet of roses that the real Randy give her (indirectly as the Ninja) as an apology. Afterwards, she dreamily walks away with hearts hovering over her head. In "Brolateral Damage", she faints at the sight of bad bro Randy.
Although Theresa doesn't know Randy is the Ninja, she seems to appreciate the Ninja immensely, too. In "Bash Johnson: 11th Grade Ninja", Theresa is part of the parade of people following Bash around when everyone thinks Bash is the Ninja. In "The Ninja Identity", after he saves her as the Ninja, she shows a lot of gratitude as she thanks and hugs him, much to Randy's delight. It isn't the only time he shows sign of her being a potential love interest, because in episode "The Brawn Also Rises" He is shocked and delighted when Theresa asks for a "Ussie." Throughout that episode, he appears to have a small crush on her and they are seen having flirty banter as well. Later in "The Ninja Supremacy, she realizes something is wrong with Howard as the Ninja, and it takes her longer than most students to embrace his more shoobtastic attitude. She is sure that the Ninja has better judgement in directions than Howard, and she praises the Ninja for saving her and her classmates in "Sorcerer in Love 2: The Sorceress's Revenge". In "Everybody Ninj-along", Theresa is disappointed by the Ninja's conduct, however, although it changes after the Ninja asks Bucky to play his song again.
Both girls are friends, seen standing next to each other in the theme song and beside Randy and Howard while hypnotized in "30 Seconds to Math". Together, they go to the Whoopee World Frightacular in "Night of the Living McFizzles". Debbie is annoyed with Theresa's attraction to Randy in the episode, rolling her eyes when Theresa calls him hardcore, and having to drag her away when the boys were heading for the cemetary. In a deleted moment from "Sorcerer in Love", both appear displeased with the boy's obsession with Amanda. In "Best Buds", they compete as a team, but they are disqualified in the first round. They are also a team for laser tag, losing to Randy and Howard and demanding their tickets back later when they figure out the boys tricked, in "Shoot First, Ask Questions Laser".
She surprisingly doesn't stay too angry at him after he hit her with his bag because he thought she was a ghost, and gave him McFizzles in order to make him stop crying in "Night of the Living McFizzles". It took her awhile to accept Howard as the Ninja during "The Ninja Supremacy", agreeing with mind-wiped Randy that he was acting wonk. She's annoyed at him for interrupting her when she's greeting Randy in "Sorcerer in Love 2: The Sorceress's Revenge". Although following Howard's orders, Theresa doesn't think he's a great leader, and doesn't seemed pleased as Randy grumpily calls Howard the real hero and leaves her with him. Like the rest of the student body, she was exasperated by the end of the episode with Howard's insistence that he knows the right way to go.NomiRandy starts flirting with Heidi instead of her in "Shloomp! There It Is!", Theresa runs away, crying. Because of this, she gets stanked, and she ends up chasing and capturing Heidi in order to get revenge on her. Heidi calls Randy the wrong name, and stanked Theresa angrily corrects her, which is something only Randy has ever done. In "Welcome Back Catfish", she is seen talking to Heidi before they both flee from a monstered Catfish Booray. They're also seen talking in the background of "M-M-M-my Bologna" before the Fly Bot causes trouble.