Katrina is the evil twin of Sabrina Spellman. Katrina lived in South Dakota before meeting Sabrina and showing her true evil colors.
When Sabrina finally solves her family secret in order to receive her Witch's License, she discovers that every member of the Spellman family is born with a twin. Afterwards, she meets her identical twin, Katrina.
Katrina and Sabrina get along very well and spend quality time together at the Spellman Family reunion in Hawaii up until they discover that they must be put on trial as one of them is the good twin and one of them is the evil twin. In the initial trial, it is shown that Katrina does various good deeds with her magic such as zapping up food for the homeless and conjuring children to read stories too and therefore, she is proven to be the good twin and Sabrina the evil one. As is customary, Katrina is told to throw Sabrina into a volcano, which she does without hesitation. However, that was a test as no good person would ever willingly push someone into a volcano and it is revealed that Katrina is actually the evil twin. Afterwards, she's sent to the prison in the "Twin Cities" in the Other Realm.
Two years later, Katrina crosses paths with Sabrina while doing community service at an Other Realm amusement park. Katrina switches her passport with Sabrina's, allowing herself to take Sabrina's place in the Mortal Realm and getting Sabrina sent to her cell in prison. While there, Katrina tries to ruin Sabrina's life such as getting Sabrina's aunts, Hilda and Zelda into a fight, and messing with Sabrina's roommates by reading Roxie's diary, ruining Miles' meteor shower observation, and making out with Sabrina's co-worker Josh even though he's dating her roommate Morgan. However, Sabrina is able to send a message to her aunts who get her out of prison and she returns, zaps Katrina back to her cell, and fixes the problems she created.
The next year, Katrina e-mails Sabrina an air head virus which turns her into a complete ditz. Unfortunately for her, Zelda is able to bribe the antidote out of her evil twin, Jezebelda, Katrina's cellmate.
- Katrina is an homage to the identical twins of other sitcom magical women, Samantha Stephens and Jeannie. In the 1960's television show Bewitched, witch Samantha Stephens has a mischievous identical cousin named Serena and in the show of the same era, I Dream of Jeannie, the titular genie has an evil identical sister with the same name.
- Coincidentally, Katrina dyed her hair auburn at the same time that Sabrina did and went back to her natural golden blonde at the same time as well.
- Also known as Nasty Girl.
Over the course of the show's seven seasons, Katrina Spellman appeared in three episodes: