Karen McCormick is the youngest member of the McCormick family. She first appeared as a background character in "Best Friends Forever". She later appeared in "The Poor Kid", where she played her first major role. Kenny is very protective of her, using his Mysterion identity to comfort her in hard times and keep her out of danger. Kenny's protectiveness extends to such a degree that she considers him her "guardian angel".
Although early press materials for Season One mentioned that Kenny had a sister, she was not seen until "Best Friends Forever". The original script for the episode "Starvin' Marvin" mentioned a sister in the final scene at the McCormick family dinner, although she was not seen in the episode itself. In "Best Friends Forever", she appeared briefly with her family at an amusement park and at the hospital visiting her brother, but she never speaks nor is she referred to. Her name first appeared on Wikipedia, attributed to Matt Stone. Her name was not confirmed until "The Poor Kid", which focused on her and Kenny's close relationship after their parents were arrested.
In her first appearance, she has dirt over much of her face, and her hair is slightly messier, but by her second appearance, she is clean besides dirt over her left eye. For pajamas, she wears a pink-collared light blue shirt with pink and purple hearts and pink bottoms or a pink one-piece with pink flowers and purple hearts. She appears to be the size of a first or second grader, though few children this age appear on the series.
Her appearance was significantly cleaned up in "The Poor Kid". Her jacket no longer has the scrapes or tears, her face is not as dirty, her hair is not as messy, and she has no mittens. In "The City Part of Town", her appearance has been slightly upgraded with her hair now tied back in pig tails and her face being messy again. Her Heidi-like hair was possibly used as a Placeholder hairstyle for animation testing.
- If Karen is around 7 years old, that means that Mrs. McCormick gave birth to her when Kenny was around 5 years old, although we never see her anywhere in the house.
- In more recent episodes, Karen is seen a lot more with her family, along with Kevin and Kenny.
Karen seems to be relatively quiet and timid, thus indicating shyness. She is almost always seen with Kenny. She is usually seen crying when dealing with stressful situations. When Mysterion is around, Karen seems to feel safer, since she does refer to him as her 'Guardian Angel'. Karen sometimes carries a pink princess doll around with her, or mostly stays around either Kenny, Kevin or her mom. It seems that Karen is lonely, and that she has very few friends, including Kevin and Kenny. In "The City Part of Town", when Kenny buys her a new doll, the first things she does is start talking to it, possibly for social reasons. Karen also seems naive as to what might harm her, as shown in"Naughty Ninjas", where she brings a syringe home.
Karen is Kenny's younger sister. She seems to have a good relationship with him, especially his alter-ego, Mysterion, to such an extent she considers him a "guardian angel". She also seems to look to Kenny for protection and comfort when she is upset; this is shown when the McCormick siblings are in The Soft Room at the police station and Karen is clutching Kenny's arm. Kenny is extremely protective of her and loves her dearly. In "The City Part of Town" he buys her a new doll with the money he earned at City Wok, much to her excitement. Karen seems to stick around Kenny whenever he's near her, and Kenny always seems to make Karen feel safe and happy.
She is Mrs. McCormick's only daughter. She seems to love her mother dearly, even after being through years of abuse likely due to her parents' fighting. She seems to go to her mother when she is scared, as shown in "The Poor Kid" when she's seen crying in her mother's arms when Stuart and Kevin are fighting. When she and her siblings are in The Soft Room, she immediately asks if she can see her mother. In "The City Part of Town", she sits on her mother's lap at the gentrification meeting and sits close to her on their couch, asking her if they can eat at the SoDoSoPa. Mrs. McCormick also started yelling about Karen's protection when she brought in a syringe from the homeless in "Naughty Ninjas".
She is Stuart's only daughter. They have not interacted much, so little is known about their relationship. She seems to be scared of him, as shown in "The Poor Kid" when she cries when he and Kevin fight, and in "The City Part of Town" when she cowers and leaves the room after he yells at her. However, Stuart shows that he does care about her, along with Kevin and Kenny, in "Naughty Ninjas" when he argues with the police about how the homeless living outside of their house are scaring the kids.
She is Kevin's sister. It is apparent that she is not as close to Kevin as she is Kenny, since they have never been shown interacting one-on-one and she cries when he and Stuart fight in "The Poor Kid". However, in "Best Friends Forever", she is seen hiding behind Kevin in the hospital. It's likely that she trusts Kevin. In "The City Part of Town", it is shown that they share a bedroom.
- "Best Friends Forever"- Seen with her family at the amusement park and visiting Kenny at the hospital.
- "The Poor Kid"- Appears as a major character. First speaking appearance.
- "The City Part of Town"- Given a baby doll by Kenny.
- "Naughty Ninjas"- Screams when a homeless man threatens Stuart and listens to his phone call to the police.
- "PC Principal Final Justice"- Seen standing with Kenny and her parents when Randy confronts the Whole Foods.
Karen can be found in the McCormick house watching TV. She says that Princess Kenny texted her about how cute New Kid is, and asks the New Kid to protect her "sister" for her. She strangely never mentions Kevin in the game.